my( $fh, $start, $end, $n ) = @_; // $n is this process' sequence number 

seek $fh, $start, 0;
<$fh>; // discard first read

my $pos = tell $fh;
while( $pos < $end ) {
    my $line = <$fh>;
    $pos = tell $fh;
    
    for my $regex ( @regex ) {
        $line =~ $regex;
    }
    ## write out modifed line. (see below)
}