I'm glad you brought up this point. I too am a huge fan of qr; however, I think this situation is a perfect use of the /o operator. I assumed that the snippet the author posted was but a morsal of his actual code; he probably uses dozens of patterns and thousands of lines of input. Compiling the regex with /o (rather than building it with qr) is ideal for this situation where a single regex is to be applied to huge amounts of data. It will result in a speed boost. For instance, I modified my earlier code and ran this benchmark:

use Benchmark;
    timethese(1000, {
        Slasho => \&withslasho,
        None   => \&without,
        qr     => \&withqr
    });

sub withslasho
{
my $str1 = 'ABCBXBCA';
my $str2 = 'APCBXBCAC';
my @array = ($str1, $str2) x 500;
my @patterns = ('B.B', 'CB')x10;
my $pat = join '|',@patterns;

foreach my $string (@array)
{
    if($string =~ /($pat)/o)
    {
        # do a pattern lookup to see which pattern matched.
        my $matched;
        foreach my $p (@patterns)
        {
            if ($1 =~ /$p/)
            {   
                $matched = $p;
                last;
            }
        }
    }
}

}

sub without
{
my $str1 = 'ABCBXBCA';
my $str2 = 'APCBXBCAC';
my @array = ($str1, $str2) x 500;
my @patterns = ('B.B', 'CB')x10;
my $pat = join '|',@patterns;

foreach my $string (@array)
{
    if($string =~ /($pat)/)
    {
        # do a pattern lookup to see which pattern matched.
        my $matched;
        foreach my $p (@patterns)
        {
            if ($1 =~ /$p/)
            {   
                $matched = $p;
                last;
            }
        }
    }
}
}
sub withqr
{

my $str1 = 'ABCBXBCA';
my $str2 = 'APCBXBCAC';
my @array = ($str1, $str2) x 500;
my @patterns = ('B.B', 'CB')x10;
my $pat = join '|',@patterns;
$pat = qr/$pat/;

foreach my $string (@array)
{
    if($string =~ /($pat)/)
    {
        # do a pattern lookup to see which pattern matched.
        my $matched;
        foreach my $p (@patterns)
        {
            if ($1 =~ /$p/)
            {   
                $matched = $p;
                last;
            }
        }
    }
}
}
[download]

Which outputs:

Benchmark: timing 1000 iterations of None, Slasho, qr...
      None: 70 wallclock secs (69.60 usr +  0.00 sys = 69.60 CPU) @ 14
+.37/s (n=1000)
    Slasho: 61 wallclock secs (61.24 usr +  0.00 sys = 61.24 CPU) @ 16
+.33/s (n=1000)
        qr: 66 wallclock secs (65.80 usr +  0.00 sys = 65.80 CPU) @ 15
+.20/s (n=1000)
[download]