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eval('&' . $action . '()');
if( $@ ) {
    die "Usage\n";
}

my $subref = \&{$action};
die "Usage" unless defined &$subref();

my %dispatch = (
                'this' => \&do_this,
...
if (defined $dispatch{$action}) {
    &$dispatch{$action};
} else { die "Usage\n"; }