Start smaller. Here is one basic way to create and use the swap sub:

use warnings;
use strict;

my @swaps = swap(2, 3);
print "@swaps\n";

sub swap {
    my $k = shift; # 1st arg
    my $l = shift; # 2nd arg
    return $l, $k; # List of 2 args swapped
}

__END__

Prints:

3 2
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• Writing subroutines
• shift
• Use parentheses after the sub name when calling. Separate args with commas.
• Basic debugging checklist