Thank you CountZero. This gave me the best solution. Turns out I was trying to open the file incorrectly as "netbpa" astutely noticed but in the end I did not have to open the file the second time. I took your shortest suggestion and used it on the string $trace as follows: 
sub t_write {
    my($file, $aOrder) = @_;
    my($trace) = $aOrder->{trace_file_data};

    if (!defined($trace) || length($trace) == 0) {
        print "No trace file.\n";
        return;
    }
    
    $trace =~ s/JOB=.*/RMT=$file/;
    open(F, "> $file\0") or die "Can't create trace-file $file ($!)\n"
+;
    print F $trace;
    close(F);
        
    #make a comment that this was done
    print "Trace file: $file\n";
}

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This solution added only one line to the original subroutine! Thank you. Warm regards,

In reply to Re^2: Change a line in a file by rightfield
in thread Change a line in a file by rightfield

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