Perl already provides a built-in function to do this. It is the 'can' function which in contained in the UNIVERSAL package which every module inherits from. From the Advanced Perl Programming Book(O'Rielly):

Chapter 7.3 UNIVERSAL

All modules implicitly inherit from a built-in module
called UNIVERSAL and inherit the following three methods:

isa (package name)

     For example, Rectangle->isa('Shape') returns true if
the Rectangle module inherits (however indirectly) from
the Shape module. 
 
can (function name)
 
     Rectangle->can('draw') returns true if the Rectangle
or any of its base packages contain a function called draw. 
 
VERSION (need version)
 
      If you say, 

          package Bank;
          $VERSION = 5.1;
 
      and the user of this module says,
 
          use Bank 5.2;

      Perl automatically calls Bank->VERSION(5.2), which
can, for instance, make sure that all libraries required
for version 5.2 are loaded. The default VERSION method
provided by UNIVERSAL simply dies if the Bank's $VERSION
variable has a lower value than that needed by the user of
the module.
 
Because Perl allows a package to shamelessly trample on
other namespaces, some packages use the UNIVERSAL module
as a holding area for some global subroutines that they
wish to export to everyone. I recommend that you do not
use this "feature" yourself (or at least not in those that
you contribute to CPAN!).