my $query = undef;
if ($query =~ /^(([\w\d\-\_]+)[:,]|)(.*)$/) {
    # we get here!
    print "$`|$1|$2|$3|$4|$'|$query|";

} else {
    exit;
}

__END__
|||||||

C:\>perl -v
This is perl, v5.8.4 built for MSWin32-x86-multi-thread
[download]

nothing matches nothing or nothing :)

C:\>perl -le"print 1 if undef =~ //"
1

C:\>perl -le"print 1 if undef =~ /|/"
1

C:\>perl -le"print 1 if undef =~ /blahblah|/"
1
[download]

What you say happens with 5.6.1 doesn't sound sounds questionable (I haven't checked the perlbug database).