Funny, I'm finding all sorts of great uses for my qr//e idea today...

  my $mac_re = do { my $tmp = join ':', ('[0-9a-f]' x 2) x 6;
                    qr/$tmp/i; };
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This is left intentionally non-anchored, so that you can use it even when searching inside a line:

  my @macs = map { m/($mac_re)/g } qx( /sbin/ifconfig -a );
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