Try localizing your variables by using my instead of our.

    my $select_job = shift;
    
    my $infile = 'input.txt';
    my $outfile = "output_$select_job.txt";

    open(my $filehandle_in, "<", $infile) or die "Can't open $infile: 
+$!";
    open(my $filehandle_out, ">", $outfile) or die "Can't open $outfil
+e: $!";
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- Miller