use strict;
use warnings;
use Switch;

my $text = 'bla fred wilma fuck fred fuck wilma bla fred fredfred';

sub replace_it {
  switch( $_[0] ) {
    case 'fred'  { return 'wilma' }
    case 'wilma' { return 'fred'  }
    die;
  };
}

$text =~ s{(fred|wilma)}{replace_it($1)}eg;

print $text;