First off, you're greping for $user on a list with the single element of ("karma.txt"). That will always return a $results of 0 (I think, or maybe undef) unless the user's name happens to be "karma" (or "karma.txt", or "kar", etc). I'm guessing that's not what you mean.

I think you're trying to avoid reading in the whole file if the user does not exist. Unfortunately, you can't <update>because you end up reading in the whole file before searching it anyway</update>. So you might as well go ahead and avoid the extra logic...

(Note! This is untested!!)

use File::Slurp;

if($msg =~ /(.*)\+\+/) {
  my $user = $1;

  my @lines = split m[$/], read_file ($karma_file);

  # Set @data to all lines not starting with $user,
  # setting $user_line as the last line starting
  # with $user as a side-effect.  Note that do{}
  # returns the value of the last expression executed,
  # so the 0 is required.
  my $user_line = "";
  my @data = grep { /^$user/ ? do{$user_line = $_;0} : 1 } @lines;

  # We already have $user, and redeclaring it causes a warning.
  # So, instead, we just skip it.
  my (undef, $karma) = split /\s/, $user_line;
  ++$karma;
  push @data, "$user $karma";

  overwrite_file($karma_file, @data);
}
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Update: I like RhetTbull and Zapawork's suggestions better. :-p

bbfu
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Nor do the wind, the sun, and the rain.
We can be like they are.