"'gt' refers to the size of the string"

No. In your code, gt looks at "dictionary sequence".

If string size/length is what you wanted, say length($str) expliciltly.

use Data::Dumper;

my @foo = ("abc", "bc");
print Dumper(sort bar1 @foo);
print Dumper(sort bar2 @foo);

sub bar1 {
    return $a gt $b;
}

sub bar2 {
    return length($a) gt length($b);
}
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