This is one big reason to indent.

If you had indented and put in some whitespace you would see:

for my $line (@lines){
  if( $Session->{'usrSystem'} )=~/$line/g;
    $Response->Write("Found a match! Hooray");
}
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Right there I can look at that code and see that the if isn't closed. And I notice that the if is missing an opening curly {.

If I just look at (instead of read) this code I can instantly tell the blocks look balanced

foreach my $line (@lines) {
    if ( $Session->{'usrSystem'} =~ /$line/g ) {
       $Response->Write("Found a match! Hooray");
    }
}
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