You're treating "filenames" as a hash
$filenames{...}
instead of a reference to a hash
$filenames->{...}
or
${$filenames}{...}

In other words,
my $file = "$filenames{$datamodel->[$count]}";
should be
my $file = $filenames->{$datamodels->[$count]};
(What's with the quotes?)

Update: ... except that $count is undefined. Maybe you want

foreach my $datamodel (@$datamodels) {
   my $file = $filenames->{$datamodel};
   print "$file\n";
}
[download]

You could also do away with $datamodels entirely, but the order of the output would be random.

foreach my $datamodel (keys %$filenames) {
   my $file = $filenames->{$datamodel};
   print "$file\n";
}
[download]

I'd get rid of $numelements. There's no use for it.

Update: Some PerlMonks references on references:
Dereferencing Syntax
References Quick Reference
References Tutorial