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    /$variable/

    /$variable/o

    use Benchmark;

...
      'string/o' => sub { $string =~ /$str_regex/o },
      'qr'       => sub { $string =~ /$qr_regex/   },
           });

Benchmark: timing 10000000 iterations of qr, string, string/o...
        qr: 11 wallclock secs (10.60 usr +  0.00 sys = 
...
          11.80 CPU) @ 847457.63/s (n=10000000)
  string/o:  7 wallclock secs ( 9.40 usr +  0.02 sys =  
           9.42 CP    U) @ 1061571.13/s (n=10000000)