sub modify {
   chomp( $_[0] );
   # whatever else happens, you've just potentially modified $_[0]
}

modify('fred');
# What would you expect to happen to 'fred'?

my $f ="fred\n";
modify($f);
print $f;
#fred

##</code><code>##

sub modify2 {
   # Create a copy of the input
   my $input = $_[0];
   chomp($input);
}

modify('fred');
# works now because we're not modifying the input, but a copy of it.

my $f ="fred\n";
modify($f);
print $f;
#fred\n