use warnings;
use strict;
use FileHandle;

my @filenames = map { $_ . '.txt' } 1 .. 3;
my @handles   = map { FileHandle->new($_, '>') } @filenames;
while (<DATA>) {
    my @cols = split /,/;
    $handles[$cols[1]-1]->print($_);
}

__DATA__
10234,1,34234,342,453,34535,3453,3464
12123,1,23432,4353,44645,45654,45645,657
2123345,2,35435,3453,454,56567,56756,567567
3234353,2,34534,4564,567567,56757,56575,24234
33453,3,45464,4564564,45645,645645,54564,56456