use strict; use warnings; use feature 'say';

for my $string (<DATA>) {
    chomp $string;
    if ( $string =~ / (?<!_) _ ([a-zA-Z]+) $ /x ) {
        say "Found $1 in $string";
    } else {
        say "No match in $string";
    }
}

__DATA__
He_is_super_hero_Spiderman
He_was_a_super_actor_Tom
He_was_a_
He_was_a_42
That_nansh_he_was_a_right__one