I'll take a closer look later when I have more time, but I didn't realize you were doing substitution regexes. For that, you either have to use eval or here are two other methods that I know how to get around the eval call.

Force the caller to pass in a code reference with the entire details of the substitution...

my $str = "This is one thing. This.";

sub cref_regex {
    my ($str, $re_cref) = @_;
    print $re_cref->($str);
}

my $cref = sub { $_[0] =~ s/This/That/ig; };

cref_regex($str, $cref);
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...or, force the caller to send in both a $search and if required, an optional $replace variable...

sub separate_regex {
    my ($str, $search, $replace) = @_;

    if (! $replace){
        print $str =~ /$search/;
    }
    else {
        print $str =~ s/$search/$replace/g;
    }
}

separate_regex($str, 'This', 'That');
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Both of those are more work for the caller, so I can't tell which is better in the long run. I've just always heard "don't use string eval", so I try to avoid it where possible.

In a recent module of mine I updated, I opted for the $search, $replace option.

-stevieb