Perl always passes by reference (not by value), so calling a sub doesn't copy the arguments.

However, the standard practice is to copy the arguments into local variables (e.g. my ($x, $y) = @_;), effectively getting copy-by-value semantics.

Since 5.20, Perl uses a copy-on-write mechanism that avoids actually copying the string until required (by the string being modified), so that copy is cheap.

sub foo {
   my ($s) = @_;   # String copied here before 5.20
   $s =~ s/.//s;   # String copied here since 5.20
}

foo($str);         # No copying here.
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So,

• It's fine to pass long strings to subs that don't modify it or copy it.
• It's fine to pass long strings to subs that don't modify it or copies of it, as long as you have Perl 5.20+