Here is a more verbose solution and assumes that your files are being stored in an array.

Not really. 

for my $file (sort grep -f, readdir $dh)

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works just as well as 
for my $file (sort @files)

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my $num = 1; is useless and detrimental. It reminds me of 

use constant TWO => 2;

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when one should rather be doing 

use constant NUM_EYES => 2;

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There's a point where constants become a hindrance. 

  my $num = 1;
  if (exists $extensions{$key}) {
    $extensions{$key} += $num;
  }
  else {
    $extensions{$key} = $num;
  }

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should be 
  if (defined $extensions{$key}) {
    $extensions{$key} += 1;
  }
  else {
    $extensions{$key} = 1;
  }

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But why not just use ++? It even works great at incrementing previously undefined values. 
  ++$extensions{$key};

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In reply to Re^2: File ext number by ikegami
in thread File ext number by Kirche

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