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# monitor.pl
while(<>) {
...
   if(/warning/) {dosomethingelse($_); }
   print; ## pass on the output
}

# monitor.pl
open(INPUT, "./script |") or die $!;
...
  print;
}
close INPUT;

# adapted from Perl Cookbook
sub filteroutput {
...
   }
   exit; #exit forked monitoring process
}