Do you want the minimum number of arguments that must be specified, or the miminum number of arguments that must be passed to the function?

# Minimum number of arguments that must be specified = 0
# Miminum number of arguments that must be passed    = 1
sub f(_) {}
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# Minimum number of arguments that must be specified
sub get_min_require_params {
   my ($sub) = @_;
   my $required_params = 0;
   my $prototype = prototype($sub);
   return 0 if !defined($prototype);
   for ($prototype) {
      if (/
         \G
         (?: \$
         |   \*
         |   \\(?: \[ [^\]] \] | [^\[] )
         |   \&
         )
      /xsgc) {
         ++$required_params;
         next;
      }

      if (/
         \G
         (?: \_   # Can only be followed by ";"
         |   \@
         |   \%
         |   \;
         |   \z
      /xsgc) {
         last;
      }

      die("Unrecognized prototype\n");
   }
}
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To get the minimum number of arguments that must be passed to the function, you'll need to move "_" to the top "if".

I don't see how either version is useful, so I'm sure there's an XY problem lurking.

perlsub (for info on prototypes), prototype