Better yet:

$criteria = "(abc|123)\$";
$teststring = "Foo walks up and says: abc";

if ( $teststring =~ /$criteria/ ) {
   print "Got $1\n";
} else {
   print "uh no, it did not\n";
}

__END__
output
======
Got abc
[download]

You might have an easier time at typing your regexp if you used qr// instead of double quotes:

$criteria = qr/(abc|123)$/;
...
[download]