#Require the file with the function definitions.
#Use eval to catch errors.
eval { require "definitions.pl"; };

#__PACKAGE__ is a token that gets replaced with the
#name of the current package. You must not quote it.
#Why? Because Perl said so.
my $package = __PACKAGE__;

#Add a 'can' test to each function call...

#This is the shortest. It uses object-oriented
#calling syntax:
top() if $package->can('top');

#If you don't like dangling if's, try this:
if ($package->can('middle')) {
    middle();
}

#This short-cuts the object-oriented calling syntax,
#and uses the relatively obscure UNIVERSAL package
#directly.
#IMO it's really messy, but it's technically the fastest.
end() if UNIVERSAL::can($package,'end');