use warnings;
use strict;

my $text= 'bla fred wilma fred wilma bla fred fredfred';
my %replace = (
    fred => 'wilma',
    wilma => 'fred',
    );
    
$text =~ s/(fred|wilma)/$replace{$1}/ge;

##</code><code>##

bla wilma fred wilma fred bla wilma wilmawilma