[~] $ perl -MBenchmark -we'timethese(-5,{x=>q{"x"=~/x/},"[x]"=>q{"x"=~/[x]/}})'
Benchmark: running [x], x, each for at least 5 CPU seconds...
       [x]:  4 wallclock secs ( 5.23 usr +  0.00 sys =  5.23 CPU) @ 852787.57/s (n=4460079)
         x:  6 wallclock secs ( 5.25 usr +  0.00 sys =  5.25 CPU) @ 1499188.19/s (n=7870738)