You can use eval and /o together for rise of performance: 
for my $day_week( qw(Mon Tue Wed Thu Fri Sat Sun) ) {
    my $regexp = "^$day_week"; 

    eval 'for my $day (qw (Mon Mon Wed Fri Sat Sun Fri Wed Tue Wed) ) 
+{               
              if($day =~ m/$regexp/o) { # do somthing }           
           }';    
}

[download]
In this case, regexp will be recompiled for each iteration of extern loop, which changes our regexp. But the regexp will not recompile every time into internal loop. It will serve. 
      
--------------------------------
SV* sv_bless(SV* sv, HV* stash);

In reply to Re: regex "o" modifier by nite_man
in thread regex "o" modifier by shemp

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