use strict;
use warnings;


package Module;

use Carp qw( croak );

my %handler = (
   foo => sub { ref $_[0] eq 'ARRAY' or croak "foo key takes a hashref";      1 },
   bar => sub { $_[0] =~ /wibble$/ or croak "bar key must end with 'wibble'"; 1 },
);

sub process_something {
   my %arg = @_;
   my @results;

   while( my ( $elem, $param ) = each %arg ) {
      push @results, $handler{ $elem }( $param );
   }
}


package main;

Module::process_something( foo => 'a' );      # <-- line 27.

##</code><code>##

foo key takes a hashref at 554371.pl line 27