I took a liberty and rephrased your question. I hope I got it right.

I am wondering how to take 2 different files whose lines have a unique key called $login. This key is at the beginning of the line and it is the only piece of information common to the records. I would like to go through the file, line by line, and concatenate the contents of all the lines with the same keys, which I will output to a third file.

To do this, you will want to read in both files, and isolate $login and use it as a hash key. (So far, so good.) So use a regular expression or split (or something) and do

 
if ( exists ( $hash{$login} ) ) {
  $hash{$login} .= $delimiter . $_;
}
else {
  $hash{$login} = $_;
}
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Then, once you are done, you will want to print out your hash.

foreach my $key (keys %hash) {
  print "$key : $hash{$key};
}
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