Try playing with this:

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
[download]

Besides reading perlretut as suggested by the Anonymous One, you should consider what you are really trying to do here. If your strings really look like this, it indicates that you are doing some awkward text munging with split and join to get to this point. There is almost certainly a better way to do it. You may be suffering from the XY Problem syndrome. You should state what your overall objective is, instead of throwing out these snippets one at a time.