Well, I was out in space, here. It's not a scoping problem at all, as others have mentioned.

Oh, well.

ReadPhoneBookFile() cannot "see" %PhoneBook, because %PhoneBook is lexical.

Okay, here is the code after I made a few changes to it.

use strict;

use vars qw/%PhoneBook/;

# Read in CSV phone book
ReadPhoneBookFile();

# Run two parameter tests
my $ValidTest = "8594";
my $InvalidTest = "9999";

print "Parameter test 1 - Number to find is: $ValidTest\n";
print 'The caller was: ', MatchPhoneNumber($ValidTest), "\n";

print "\nParameter test 2 - Number to find is: $InvalidTest\n";
print 'The caller was: ', MatchPhoneNumber($InvalidTest), "\n";

exit;


sub ReadPhoneBookFile
###
###  Reads in a CSV separated phone book, splits, stuffs details into 
+a hash
###
###  In real-world script, data is read from an external file.
{
    foreach (<DATA>) {
        my @Details        = split(/,/,$_);
        $PhoneBook{$Details[0]} = $Details[1];
    }
}


sub MatchPhoneNumber
###
### Attempts to match the phone number with a name from the phonebook
### Either returns the caller name if number in hash or "Unknown calle
+r".
###
{
    my $NumToFind = shift;

    return defined $PhoneBook{$NumToFind}
      ? $PhoneBook{$NumToFind}
      : 'Unknown Caller';
}

__DATA__
8594,Tom
9000,Dick
1234,Harry
[download]

A few notes:

• You had 8954 in some places, and 8594 in others
• As noted above, %PhoneBook was lexical, therefore not in scope when ReadPhoneBookFile() and MatchPhoneNumber() were called.
  • We use use vars to put %PhoneBook in the symbol table for this scope and keep use strict happy.
  • We could also just pass %PhoneBook to the functions. Many would argue this is a better way...
• I removed many of your temporary variables. This is mainly just an issue of performance and style. No offense...

So, it was just a scoping issue...

Russ
Brainbench 'Most Valuable Professional' for Perl

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