You should probably be using a hash in the first place. Try something like this:

sub addfor{
  my ($mainwidget,$entryVariable) = @_;
  my %history;

  if (exists $history{$entryVariable}) {
    $mainwidget->messageBox
      (-icon => 'error',
       -type => 'OK',
       -title => 'Error for input',
       -message => "duplicate input");
  } else {
    $widget{'forwardlistbox'}->insert
      ('end', $entryVariable);
    $history{$entryVariable} = 1;
  }
}
[download]

Hashes are almost always the proper way to implement a set.

(Also, 'use strict;'! You've got some phantom variables floating around. I'm not sure whether $widget is the same as $mainwidget or not, and @history was unscoped.)