I suspect that you have a regex with a fixed a variable part.

use strict;
use warnings;
my $string = 'title: ABC_II';
foreach my $index (qr/I/, qr/II/, qr/III/) {
    print "Match\n" if $string =~ m/ABC_$index/;
}
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You want to know if you can speed it up by pre-compiling the fixed part.

use strict;
use warnings;
my $string = 'title: ABC_II';

my $fixed = qr/ABC_/;
foreach my $index (q(I II III)) {
    print "Match\n" if $string =~ m/$fixed$index/;
}
[download]

Clearly the syntax is allowed. You would have to benchmark your case to find out if and how much it helps.