If you don't mind consuming the input, you can use an anchored regex to eat the input as you generate the summary.

my %mapping = 
  (
  '[a-zA-A0-9]+' => 'A',
  );

my $summary = '';

CHUNK: while (length $filedata)
{
  foreach my $reg (keys %mapping)
  {
    if ($filedata =~ s/^$reg//)
    {
      $summary .= $mapping{$reg};
      next CHUNK;
    }
  }
  die "Chunk starting with '". substr($filedata, 0,10) . "' did not ma
+tch any rules!";
}
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If I'm confused as to your goal and you actually want to do the opposite of this, you can take the reverse of the %mapping hash and concatenate a big regex string by looking up the regex substring for each character in the summary in turn.