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;
}
}
}
}