Dog, Table, 12:30, Mercury,abcdef
Dog, Chair, 5:30, Mercury,abcdef
Dog, Table, 12:30, Venus,abcdef
Pig, Door, 3:30, Earth, 123
Pig, Door, 3:30, Earth, 45678943985
Goat, Couch, 8:45, Mars, 9876


##</code><code>##

{Dog.txt}
Dog, Table, 12:30, Mercury,abcdef
Dog, Chair, 5:30, Mercury,abcdef
Dog, Table, 12:30, Venus,abcdef

{Pig.txt}
Pig, Door, 3:30, Earth, 123
Pig, Door, 3:30, Earth, 45678943985

{Goat.txt}
Goat, Couch, 8:45, Mars, 9876


##</code><code>##

#!/usr/bin/perl -w
use strict;

my $sourcefile = $ARGV[0];
my $currentline;
open(my $fh, '<', $sourcefile) or die ("Could not open $sourcefile");
#gets the filename from the cmd line

while(my $line = <$fh>) #until EOF, do the following
    {
        my $nextline =' ';
        #initialize the next line to empty before starting
         foreach my $line (<$fh>) #for every line go in
            {             
                chomp($line);
                my @currentline = split(/,/, $line); #split the fields
                next if (($currentline[0]) eq (@nextline[0]));
                #if 1st field in currentline = 1st field in the 2nd line
                open OUT, ">> $currentline[0].txt" or
                die "Could not open file $currentline[0].txt for output.\n$!";
                #create the filename based upon the first field contents
                #if the file already exists, append to it
                print OUT $currentline;
                #throw everything in that particular line into the file
                $nextline = $currentline; #go to the next line
                close OUT; #close the file
            }
    }