I've done some benchmarking of the three options (not including lima1's suggestion of suffix-trees): sequential regular expressions, sequential index's, and merging the regular expressions together using Regexp::Assemble. I'm not great at writing benchmarks, so perhaps this script is simplistic, and I'd appreciate comments on how to improve it. Nevertheless, here are the results:

Regexp : 230 wallclock secs (229.86 usr +  0.06 sys = 229.92 CPU)
Index  : 22 wallclock secs (21.47 usr +  0.01 sys = 21.48 CPU)
Merged : 663 wallclock secs (663.73 usr +  0.17 sys = 663.90 CPU)
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So it appears that even without forking the indexing approach is much faster for exact matches. Here's the benchmarking code I used.

use strict;
use Bio::SeqIO;
use Benchmark;
use Regexp::Assemble;
use Carp;

my $seqio = Bio::SeqIO->new(
        -file => "chrY.fa.masked",
        -format => "fasta"
        );
my $seq = $seqio->next_seq();
my $sequence = $seq->seq();

my @strings = (
        'CACGTG',
        'GTGCAC'
        );

my $t0 = new Benchmark;
for (1..100) {
        foreach my $string (@strings) {
                my $len = length($string);
                while ($sequence =~ /$string/g) { my $val = pos($seque
+nce) - $len; }
                }
        }
my $t1 = new Benchmark;

my $t2 = new Benchmark;
for (1..100) {
        foreach my $string (@strings) {
                my $pos = 0;
                my $len = length($string);
                while ( ($pos = index($sequence, $string, $pos)) >= 0 
+) {
                        my $val = substr($sequence, $pos, $len);
                        $pos += $len;
                        }
                }
        }
my $t3 = new Benchmark;

my $t4 = new Benchmark;
for (1..100) {
        my $ra = Regexp::Assemble->new();
        $ra->add(@strings);
        while ( $sequence =~ /$ra/g) { my $val = pos($sequence) - 6; }
        }
my $t5 = new Benchmark;

my $td1 = timediff($t1, $t0);
my $td2 = timediff($t3, $t2);
my $td3 = timediff($t5, $t4);

print 'Regexp : ', timestr($td1), "\n";
print 'Index  : ', timestr($td2), "\n";
print 'Merged : ', timestr($td3), "\n";
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