It seems to me that perl copies a string whenever you do anything, regardless of whether you really modify the copy later or not?
No, perl copies strings on assignment ($foo = $bar). Example:

#!/usr/bin/perl --

use strict;
use warnings;

my $str  = "foo";
my $copy = $str;

print "$copy $str\n";

nocopy($str);
yescopy($str);

print "$copy $str\n";    # now $str is foobar

sub nocopy { $_[0] .= 'bar'; '...' }
sub yescopy { my ($copy) = $_[0]; $copy = shift; $copy .= 'bar' }

__END__
foo foo
foo foobar
[download]

that you can hint PERL that you will not modify some variable and thus it removes the extra baggage that is otherwise required if you were to modify it etc?

You are probably thinking of constant and/or making variables readonly, which forces you not to write code that try to modify those variables, as a result there is no unneeded copying

... PERL ...

The Perl community likes to call it Perl, or perl, but not so much PERL, even if its a fun bacronym generator.