Maori Farewell

2024-07-07

This page explains the Maori Farewell trick, which gives you a BEGIN block inside a Perl one-liner. The butterfly operator handles cleanup at the end, but this trick handles setup at the start.

-M5;

That's it. The -M switch, the number 5, and a semicolon. Everything after the semicolon runs before your main code.

Named for the Maori people of New Zealand (the pattern continues the cultural naming tradition of the Eskimo greeting). It exploits -M in a way the designers probably didn't intend.

Part 1: THE TRICK

The -M switch normally loads a Perl module:

perl -MJSON -e 'print encode_json({a=>1})'

But with -M5;, something different happens:

perl -M'5;print "HELLO\n"' -ne 'print' file.txt

The command produces this output:

HELLO
line 1 from file
line 2 from file
...

The print happens BEFORE the file processing starts. It acts as a pseudo-BEGIN block hiding inside the -M switch.

Part 2: HOW IT WORKS

When Perl sees -M5;, it interprets this as:

use 5;          # Require Perl version 5 (always true)
print "...";    # Execute this code

The semicolon separates the version requirement from arbitrary code. Since we are all running Perl 5 or newer, the version check passes silently.

What comes after the semicolon runs at compile time, before the -n or -p loop even starts. It is not quite a real BEGIN block, but it acts like one for practical purposes.

      .--.
     |o_o |
     |:_/ |
    //   \ \
   (|     | )
  /'\_   _/`\
  \___)=(___/

Part 3: PRACTICAL EXAMPLES

Print a timestamp before processing:

perl -M'5;print scalar localtime, "\n"' -ne 'print if /error/i' log.txt

The command produces this output:

Wed Jan  5 15:30:45 2025
[15:30:46] ERROR: Database connection failed
[15:31:02] ERROR: Invalid user input

The timestamp prints first, then matching lines follow.

Set a custom record separator:

perl -M'5;$/="\n\n"' -ne 'print if length > 100' document.txt

This sets paragraph mode BEFORE the loop starts. Each "line" is now actually a paragraph (text between blank lines).

Initialize a counter with a starting value:

perl -M'5;$n=100' -lne 'print $n++, ": $_"' file.txt

The command produces this output:

100: first line
101: second line
102: third line

The counter starts at 100 instead of 0.

Part 4: WHY NOT JUST USE BEGIN?

You might think:

perl -ne 'BEGIN { print "hello\n" } print' file.txt

And you would be right. That works too. But compare the character counts:

-M'5;print "hi"'           17 characters
'BEGIN{print "hi"}'        19 characters

In code golf, every character matters. The Maori Farewell saves two.

It also feels different because the initialization sits visually apart from the main loop code, which some find cleaner:

perl -M'5;$/=undef' -M'5;$verbose=1' -ne '...'

You can stack multiple -M flags, and each one handles one setup task. The command reads like a config file.

Part 5: VERSION FEATURES

Here is a bonus trick. You can use a real version number to enable language features:

perl -M5.010 -E 'say "hello"'

The -M5.010 flag enables Perl 5.10 features, including say. You can combine it with the semicolon trick:

perl -M'5.010;$count=0' -nE 'say ++$count, ": $_"' file.txt

You get both version features and initialization code in a single flag.

Common version features:

5.010    say, state, given/when
5.014    /r modifier for non-destructive substitution
5.016    __SUB__ for anonymous recursion
5.020    Postfix dereferencing

Part 6: LIMITATIONS

The Maori Farewell does not work with module import lists:

# THIS FAILS:
perl -M"List::Util=sum;print 'hi'" -e '...'

The = syntax for selective imports breaks the trick. If you need module functions, load the module normally:

perl -MList::Util=sum -M'5;print "hi\n"' -e '...'

You use two -M flags instead. One flag loads the module, and the second flag runs the farewell.

Part 7: COMBINING WITH BUTTERFLY

Now you have both ends covered:

perl -M'5;print "START\n"' -lne '$sum+=$_ }{ print "Sum: $sum"' nums.txt

The command produces this output:

START
Sum: 150

The Maori Farewell handles the beginning. The butterfly operator handles the end. Your one-liner now has a proper setup and teardown.

Part 8: THE ANNOTATED VERSION

perl -M'5;print "START\n"' -lne '$sum += $_ }{ print "END: $sum"' file

Breaking it down:

PIECE                       WHAT IT DOES
------------------------    ----------------------------------
perl                        Run Perl
-M'5;print "START\n"'       Maori Farewell: runs before loop
-l                          Auto-chomp input, add newlines
-n                          Wrap code in while(<>) loop
-e '...'                    The main code
$sum += $_                  Loop body: accumulate sum
}{                          Butterfly: end loop, start end block
print "END: $sum"           End block: print final summary
file                        Input file

You can chain three tricks into one command:

1. Maori Farewell for setup
2. -n for implicit loop
3. Butterfly for cleanup

Part 9: REAL-WORLD USES

Print column headers before CSV processing:

perl -M'5;print "Line,Content\n"' -lne 'print "$.,\"$_\""' data.txt

Set environment variables before processing:

perl -M'5;$ENV{DEBUG}=1' -ne '...' file.txt

Initialize complex data structures:

perl -M'5;%seen=();@order=()' -lne '
    push @order, $_ unless $seen{$_}++;
}{ 
    print for @order
' file.txt

That last example is a unique-lines-in-order filter with explicit initialization of the tracking structures.

Part 10: THE NAME

Why "Maori Farewell"? It continues the cultural greeting theme:

}{     Eskimo greeting (two noses touching)
-M5;   Maori farewell (hongi - pressing foreheads)

Both names reference traditional greetings from indigenous peoples. The Perl community has a habit of using whimsical names.

Whether the cultural references are appropriate is debatable. The names stuck because they are memorable, and that matters for tribal knowledge passed between Perl hackers.

                  .---.
                 /     \
                | START |
                 \     /
                  '---'
                    |
                    V
                .-------.
               /  LOOP   \
               \  -n -p  /
                '-------'
                    |
                    V
                  .---.
                 /     \
                |  END  |
                 \     /
                  '---'

    Maori at the top, Butterfly at the bottom

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