split, join, and splice are your friends:

my @accounts = split /\n/, <<EOT;
1,joe,123 First St.
2,mary,234 Second St.
EOT

my @purchases =  split /\n/, <<EOT;
1,222,veg,01/05/05
1,444,fruit,02/04/05
1,555,bread,03/09/06
1,777,butter,05/07/05
1,888,spice,09/06/07
1,999,coffee,02/06/05
2,444,veg,01/01/06
EOT

my $accounts_by_id = {};

for (@accounts) {
    my($id, $rest) = split /,/, $_, 2;
    $accounts_by_id->{$id}->{info} = $rest;
}

for (@purchases) {
    my($id, $rest) = split /,/, $_, 2;
    push @{$accounts_by_id->{$id}->{purchases}}, $rest;
}

for (@accounts) {
    my($id, $rest) = split /,/, $_, 2;

    {
        my @five = splice
               @{$accounts_by_id->{$id}->{purchases}}, 0, 5;
        last unless @five;
        print "$id,$accounts_by_id->{$id}->{info},",
              join(',', @five), "\n";
        redo if @five == 5;
    }
}
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