But this does not work where the brackets contain only text.

Works for me:

#!/usr/bin/env perl 
use strict;
use warnings;

use Test::More;

my %foo = (
    'a(b)c' => 'ac',
    'ab()cd' => 'abcd'
);
my $snr = qr/\(.*?\)/;

for my $have (keys %foo) {
    my $want = $foo{$have};
    $have =~ s/$snr//sg;
    is ($have, $want);
}
done_testing ();
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