/o can still be useful from time to time. Firstly, you occasionally expect that the interpolated variable may change but still want to use the original value (but this is rare). Secondly, the "recompile only if changed" logic requires checking whether the pattern has changed, which takes time:

  perl -MBenchmark -w
    ($s, $t) = qw/ foo xfoox /;
    timethese(-1, {
      oful => q{ $t =~ /$s/o },
      oless => q{ $t =~ /$s/ },
    })
  __END__
  Benchmark: running oful, oless for at least 1 CPU seconds...
        oful:  2 wallclock secs ( 1.08 usr +  0.00 sys =  1.08 CPU) @ 
+2427258.33/s (n=2621439)
       oless:  2 wallclock secs ( 1.15 usr +  0.00 sys =  1.15 CPU) @ 
+1841144.35/s (n=2117316)
[download]