If the return from the method call is the only thing you want to test against, just match against that:

$_ =~ C->flag();
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If you need to have specific matches within the match (I mean $1, $2 or whatever), you'd have to construct the regex in the method.

sub flag {
  return '(.)foo(\\d+)bar'
}
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$1 and $2 would be available.
You can't pad either side of the match by surrounding it with // and adding things in, because that just wouldn't work.

Jasper