Instead of using ParseRecDescent, may I suggest a Regexp::Common solution? Parsing lists is one of its options.

#!/usr/bin/perl

use strict;
use warnings;

use Regexp::Common;

my $re = $RE{list}{-sep => '_'}{-pat => '(?:(?!_)\w)+'};

while (<DATA>) {
    chomp;
    print "'$_' ", /^$re$/ ? "matched\n" : "did not match\n";
}


__DATA__
apple_pear
apple_plum_grape
_
apple
_apple
apple_
apple__pear




'apple_pear' matched
'apple_plum_grape' matched
'_' did not match
'apple' did not match
'_apple' did not match
'apple_' did not match
'apple__pear' did not match
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Abigail