... open file handles, etc. ...

defined(my $first_line = <RAW>) or die "reading raw input: $!";
chomp $first_line;
my ($first_line_first_field) = split m{ , \s* }xms, $first_line;

while (defined(my $following_line = <RAW>)) {
    chomp $following_line;
    my ($following_line_first_field) = split m{ , \s* }xms, $following_line;
    compare($first_line_first_field, $following_line_first_field);
    }

close RAW or die "closing raw handle: $!";