#! perl -w
use strict;

my $haystack = "abcdefghijklmnopqrstuvwxyz";
my @needles = qw(abc cde fgh ghi ijk pqr rst xyz);

my $regex = '(' . join('|', @needles ) . ')';
print "Pre-compilation:", $regex,$/;

$regex = qr/$regex/s;
print "Post-compilation:", $regex, $/;

$haystack =~ /$regex/g;

if ($#-) {
    print "$#+ matches found", $/;
    for (1..$#+) {
        print "\$_ is $_, & \$1 is $1", $/;
        print  "$haystack contains ${$_} at $-[$_]", $/;
    }
}

__END__
C:\test>test
Pre-compilation:(abc|cde|fgh|ghi|ijk|pqr|rst|xyz)
Post-compilation:(?s-xim:(abc|cde|fgh|ghi|ijk|pqr|rst|xyz))
1 matches found
$_ is 1, & $1 is abc
Can't use string ("1") as a SCALAR ref while "strict refs" in use at C:\test\test.pl
line 19.

C:\test>