#!/usr/bin/perl
use strict;
use warnings;

my %seen;
while (<DATA>) {
    chomp;
    my ($acct, $data) = $_ =~ m{^(\d\d)(.*)};
    next if $seen{$acct}{$data}++;
    append_data($acct, $_);
}

# If you know you are not going to exceed the open filehandle limit
# You can improve performance by caching filehandles
sub append_data {
    my ($acct, $line) = @_;
    open(my $fh, '>>', "$acct.txt") or die "Unable to open '$acct.txt' for appending: $!\n";
    print $fh $line;
}