You could use grep:

sub addfor{
   my ($mainwidget,$entryVariable) = @_;
   unless (grep { $_ eq $entryVariable } @history) {
      $widget{'forwardlistbox'}->insert(end => $entryVariable);
      push @history, $entryVariable;
   }
}
[download]

This will be slower than necessary since it will keep testing after $entryVariable is found. A faster version could use first() from List::Util:

use List::Util 'first';
sub addfor{
   my ($mainwidget,$entryVariable) = @_;
   unless (first { $_ eq $entryVariable } @history) {
      $widget{'forwardlistbox'}->insert(end => $entryVariable);
      push @history, $entryVariable;
   }
}
[download]

Note that your code may have problems: I don't see where @history is getting initialized and you're not using $mainWidget in the subroutine. I don't know if these are real problems or not but they might be.

-sam