use strict;
use Switch;
my $text="bla fred wilma fuck fred fuck wilma bla fred fredfred";
$text =~ s{(fred|wilma)}
	{
		my $replace_text;
		if($1 eq "fred") {
			$replace_text='wilma';
		}
		else {
			$replace_text='fred';
		};
		$replace_text;

	}eg;
print $text;

##</code><code>##

use strict;
use Switch;
my $text="bla fred wilma fuck fred fuck wilma bla fred fredfred";


$text =~ s{(fred|wilma)}
	{
		my $replace_text;
		switch($1) {
			case 'fred' {$replace_text='wilma'}
			case 'wilma' {$replace_text='fred'}
		};
		$replace_text;

	}eg;
print $text;