use Benchmark 'cmpthese'; $iterations = 1000000; %re = ( foo => 'foo1', bar => 'bar1', ); $re = join '|', keys %re; $re = qr/($re)/; $name1 = "RE"; $code1 = << 'END_CODE1'; $_ = 'foo bar'; s/$re/$re{$1}/g; END_CODE1 $name2 = "Loop"; $code2 = << 'END_CODE2'; $_ = 'foo bar'; for my $sub( keys %re ) { s/$sub/$re{$sub}/g; } END_CODE2 cmpthese( $iterations, {$name1 => $code1, $name2 => $code2} ); __END__ Benchmark: timing 1000000 iterations of Loop, RE... Loop: 24 wallclock secs (24.77 usr + 0.00 sys = 24.77 CPU) @ 40377.94/s (n=1000000) RE: 8 wallclock secs ( 7.27 usr + 0.00 sys = 7.27 CPU) @ 137551.58/s (n=1000000) Rate Loop RE Loop 40378/s -- -71% RE 137552/s 241% --