I've had a look at your pretty solution (with qr). However, it currently does not work, for several reasons.

The first in the one you mention : it does not compile, because of qr and (?{...}) block. That is explained in perlre, about the (?{...}) block :

For reasons of security, this construct is forbidden if the regular expression involves run-time interpolation of variables, unless the perilous use re 'eval' pragma has been used (see re), or the variables contain results of qr// operator (see perlop/"qr/STRING/imosx").

In short, that is to prevent a external variable containing such a block to be executed in the regex engine. The solution is described in the doc : use re 'eval'

However, some other thing does not work yet : when the regex qr/(?:(\Q$_\E))(?{$result{$_}++})/ is matched, that does not set the %result hash entry, because at execution time, $_ is not what you expect. If you look at the regex created :

print $re;
__DATA__
(?-xism:(?-xism:(?:(foo))(?{$result{$_}++}))|(?-xism:(?:(bar))(?{$resu
+lt{$_}++})
)|(?-xism:(?:(12345))(?{$result{$_}++}))|(?-xism:(?:(abcd))(?{$result{
+$_}++})))
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you'll see that $_ variable is not replaced by the keyword values. To my understanding, it is because (?{...}) contains code that will be interpreted later, so perl does not interpret in at that time, leaving it unmodified.

So, to do what we want here, I use (...) to catch a execution time the match and set the correct hash entry. There is a final problem : the name of the variable.

my $data = "foofdsfdsbar";
my @matches = 
    $data =~ m/(foo)(?{print "($1,$2,$+)"})|(bar)(?{print "($1,$2,$+)"
+})/g;
__DATA__
(foo,,foo)(,bar,bar)
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That is to say : $+ is the only one that surely contains the last pattern matched (BTW, I know about performance penalty when using it, but I won't care for now ;)

So, let's code it !

#!/usr/bin/perl

use strict;
use re 'eval';

my @keywords = qw/foo bar 12345 abcd/;
my ($string) = "foobarfoo1234523423412345abcdefsadfabc";
our %result;

my @regexen = map {
    qr/(?:(\Q$_\E))(?{$result{$+}++})/
} @keywords;

my $re = do {
    local $" = '|';
    qr/@regexen/;
};

my @match = $string =~ /$re/g;

print "$_: $result{$_}\n" for @keywords;
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