File ext number

Kirche has asked for the wisdom of the Perl Monks concerning the following question:

I'm trying to count number of files for each extension in a folder:

opendir H, './';
$ext{$1} += (m/([^.]+)$/) for grep { -f } readdir H;
closedir H;

print "$_ - $ext{$_}\n" for keys %ext;
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I'm getting following error: Use of uninitialized value in hash element at line 4. But if i replace += with = it shows no error. What i'm doing wrong?

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