Thanks grinder.

I had already used module Regexp::Assemble for another quite similar script, and I had thought of using it with track=>1 for my replacement script. But I could not because my new script hanged whenever I created a Regexp::Assemble object with track=>1.

The exact line in my code where the script hanged:

my $Rpatt = Regexp::Assemble->new( chomp=>1, track=>1 )->add( keys(%PA
+TTS) );
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Where %PATTS is a hash with elements of the form 'pattern->replacement'. Any idea why it hanged?