unpack is great for fixed width fields.

# @fields = unpack('a5 a6 a7 a5', $_);
# @fields = unpack('a4 x1 a5 x1 a6 x1 a5', $_);

open(my $fh_in, '<', ...)
   or die("Can't open input file: $!\n");

while (<$fh_in>) {
   chomp;

   my ($file_name, $rest) = unpack('a4 x1 a*', $_);
   $file_name .= '.out';

   open(my $fh_out, '>>', $file_name)
      or die("Can't open $file_name for append: $!\n");

   print $fh_out ("$rest\n");
}
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By the way, your data is not fixed width. The 5th record is shorter.