MAIN_LOOP: is a label and is the target of constructs like next MAIN_LOOP; and last MAIN_LOOP;. However, unless you have nested loops and need "next" or "last" to apply to the outer loop, labels very seldom need to be used. Consider:

use strict;
use warnings;

MAIN_LOOP:
for my $number_to_check (1 .. 20) {
    for my $colour ('red', 'green', 'black') {
        next MAIN_LOOP if $number_to_check % 3 and $colour eq 'green';
        print "$colour-$number_to_check ";
    }

    print "\n";
}
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prints:

red-1 red-2 red-3 green-3 black-3 
red-4 red-5 red-6 green-6 black-6 
red-7 red-8 red-9 green-9 black-9 
red-10 red-11 red-12 green-12 black-12 
red-13 red-14 red-15 green-15 black-15 
red-16 red-17 red-18 green-18 black-18 
red-19 red-20
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