I am not sure what you are trying to accomplish. But I do find the following code rather strange. $pattern actually makes no difference here because the value of $pattern is never used in a statement related to output. I figure you could delete all lines involving $pattern.

my($pattern) = 0;

while (<INPUT>) {
   if ($_ =~ s/$searchPattern1/$replacePattern1/g) {
   $pattern = 1;
   }
   elsif ($_ =~ s/$searchPattern2/$replacePattern2/g) {
   $pattern = 2;
   }
   elsif ($_ =~ s/$searchPattern3/$replacePattern3/g) {
   $pattern = 3;
   }
   print OUTPUT $_;
}
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