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use strict;
use Benchmark qw(cmpthese);
...
  'chars'   => sub { $foo =~ /[Aa][Bb][Cc]/og },
  'charvar' => sub { $foo =~ /$re/og },
});

Benchmark: timing 1000000 iterations of chars, charvar, i...
     chars:  2 wallclock secs ( 1.97 usr +  0.00 sys =  1.97 CPU) @ 50
+7614.21/s (n=1000000)
...
charvar 492611/s      --     -3%    -35%
chars   507614/s      3%      --    -34%
i       763359/s     55%     50%      --