Either use a dictionary that already has plurals in it, or use the following preprocess function:

use Lingua::EN::Inflect qw /PL/;
sub preprocess {
    my $file = shift;
    open my $fh => $file or die "Failed to open $file: $!\n";
    local $_;  
  LINE:
    while (<$fh>) {
        chomp;
        $_ = lc;
        next if /[^a-z]/;
      WORD:
        foreach ($_, PL ($_)) {
            $valid {$_} = 1;
            while (length) {
                next WORD if $prefix {$_} ++;
                chop;
            }
        }
    }
}
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Abigail