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use warnings;
use strict;

my @array = (1..20);
my @array3 = sort { $a <=> $b } map { $_ % 5 } @array;
print join(', ', @array3);

Everyone knows that debugging is twice as hard as writing a program in the first place. So if you're as clever as you can be when you write it, how will you ever debug it? -- Brian Kernighan: "The Elements of Programming Style"