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use strict;
use warnings;
...
    printf qq(%-7s => "%s"\n), qq("$t"), swap( $t );
}
exit( 0 );

sub swap {
    my ($str) = @_;
...
    }
    return $str;
}

""      => ""
"1"     => "1"
...
"123"   => "213"
"1234"  => "2143"
"12345" => "21435"