Once means once. The regex is evaluated the first time
the line is processed then Perl never checks it again.

The exceptional case is when you use eval
this lets you optimize and be flexible both. You can
get your $regex value as per usual, but use it
inside an eval. This can be a big win.

for ( XXX some loop ) {
   $patt = gen_patt( $blah);
   eval 'for $i ( @lines) {
             if ( $i =~ m/$patt/o){
                  process( $i);
             }
         }';
   die "eval error" if $@;
}
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