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

...

sub getToken {
    my $output = `curl -k -s -XPOST (URL) -dgrant_type=password -dclient_id=(client ID) -dclient_secret=(client secret) -dusername=(username) -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}
}