If you must do this, try like this instead:

use warnings;
while (<FH> ) {
    chomp;
    if ( /Impressions:/ ) {
       my($text, $value) = split(/:/, $_);
       print "Impressions: $value";
       print "\n";
       {
          no warnings "numeric";
          $value += 0;                 # turn $value into a number
       }
       $impressions += $value;
    }
}
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