Hello bestfa, and welcome to the Monastery!

Please put your code in <code> tags — see Markup in the Monastery

If you’re reading the input file line-by-line, you’ll want to create filehandles on the fly, one for each unique data type. And when you open each output file, you’ll have to open it for appending (>>), not writing (>), to allow successive iterations of the loop to add new entries to the end of the file.

The data structure you need is a hash, not an array. Specifically, a hash that maps output file names data types to their corresponding filehandles. Then you can use Perl’s built-in exists function on the hash to determine whether a given data type already has an associated filehandle. Here is how I would approach this task:

#! perl
use strict;
use warnings;

my $in_file = 'input.txt';
my %out_files;

open(my $in, '<', $in_file)
    or die "Cannot open file '$in_file' for reading: $!";

while (my $line = <$in>)
{
    my ($datum, $type) = split ' ', $line;

    unless (exists $out_files{$type})
    {
        my $filename = $in_file . $type . '.txt';

        open(my $fh, '>>', $filename)
            or die "Cannot open file '$filename' for appending: $!";

        $out_files{$type} = $fh;
    }

    print { $out_files{$type} } $datum . "\n";
}

for (keys %out_files)
{
    close $out_files{$_}
        or die "Cannot close the '$out_files{$_}' output file: $!";
}

close $in
    or die "Cannot close file '$in_file': $!";
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Update: Fixed die message for closing output files; also corrected fourth paragraph.

Hope that helps,