Hello ardibehest, and welcome to the Monastery!

As LanX says, once you’ve opened a file you need to access it via the filehandle, not the filename. In addition, you should get into the habit of using the 3-argument form of open, using lexical variables instead of barewords for filehandles, calling close on the filehandle when the file is no longer being used, and always checking open and close for success or failure:

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

use constant
{
    DATA_FILE      => 'try.txt',
    MATCHED_FILE   => 'matched.txt',
    UNMATCHED_FILE => 'unmatched.txt',
};

open(my $input,     '<', DATA_FILE)
    or die "Cannot open file '" . DATA_FILE      . "' for reading: $!"
+;
open(my $matched,   '>', MATCHED_FILE)
    or die "Cannot open file '" . MATCHED_FILE   . "' for writing: $!"
+;
open(my $unmatched, '>', UNMATCHED_FILE)
    or die "Cannot open file '" . UNMATCHED_FILE . "' for writing: $!"
+;

while (my $line = <$input>)
{
    chomp $line;

    my ($left, $right) = split /=/, $line;
    my  @left_array    = split / /, $left;
    my  @right_array   = split / /, $right;

    my  $left_count    = scalar @left_array;
    my  $right_count   = scalar @right_array;

    if ($left_count == $right_count)
    {
        print $matched "$line\n";
    }
    else
    {
        my $diff = abs($left_count - $right_count);
        print $unmatched $line, "($diff) \n";
    }
}

close $unmatched or die "Cannot close file '" . UNMATCHED_FILE . "': $
+!";
close $matched   or die "Cannot close file '" . MATCHED_FILE   . "': $
+!";
close $input     or die "Cannot close file '" . DATA_FILE      . "': $
+!";
[download]

Some notes:

• Your line my @right_array = split / /, $right: ends in a colon, not a semicolon. This is a syntax error.
• Likewise, the line my $diff = abs (left_count - $right_count) lacks both a terminating semicolon and a $ sigil (prefix) on the variable $left_count.
• Your use of chomp is correct, but I think it’s better practice to chomp first thing in the loop and so keep the contents of $line the same throughout the loop body. (This will be less confusing for the maintenance programmer — likely you! — trying to understand the code some time in the future.)

It’s great that you use strict and warnings. Here are some additional pragmas you will find handy:

• diagnostics: Useful for understanding errors and warnings messages.
• constant: See the above code.
• autodie: A convenient alternative to explicitly testing for success on file open and close.

As you are new to Perl, be sure to check out chromatic’s book Modern Perl, which is available for free download at http://modernperlbooks.com/mt/index.html.

Hope that helps,

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