spock:~ [13:01:13]$ cat guyov.pl ; ./guyov.pl
#!/usr/bin/perl

use strict;
use warnings;
use 5.010;

$_ = "foo bar baz";

s/ (?<first> \w+)\s+(?<second> \w+)
 / $+{first} =~ y|o||d; "$+{first} $+{second}" /ex;

say;

__END__
Modification of a read-only value attempted at ./guyov.pl line 9.