$output =~ /"access_token":"(.*)", "ex/g;
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You never check if the output really matches.

Most likely, if you check, then you find that it doesn't match:

$output =~ /"access_token":"(.*)", "ex/g
    or die "Couldn't find access token in [[$output]]";
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But maybe you really do want to parse the output in a better way, since it seems as if the output could be JSON:

use JSON 'decode_json'; # or JSON::XS , or JSON::Tiny

...

sub getToken {
    my $output = `curl -k -s -XPOST (URL) -dgrant_type=password -dclie
+nt_id=(client ID) -dclient_secret=(client secret) -dusername=(usernam
+e) -dpassword=(password)`;
    my $decoded_output = decode_json( $output );
    die "Couldn't find access token in [[$output]]"
        unless $decoded_output->{access_token};
    return $output->{access_token}
}
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