The bottom line is: "Don't use switch - it has issues".

Often you can use a dispatch table instead, although in your sample a simple hash is all that is required:

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;
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Prints:

bla wilma fred wilma fred bla wilma wilmawilma
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