Since in this type of situation I'd normally expect the one pattern to be matched against many strings, I'd usually aim to approach this instead by modifying the regexp:

  my @ignore=(' ','-'); #whatever
  my $ignoreclass = sprintf '[%s]', join '', map quotemeta, @ignore;
  $re = join $ignoreclass, split //, $re;
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Of course this is only so simple if the initial pattern is a simple string: a full-on regexp is rather more difficult to introduce such modifications to reliably.

Hugo