my $out_fh;
while (<$in_fh>) {
   if (my ($out_fn) = /^< (.*)/) {
      open($out_fh, '>', $out_fn)
         or die("Unable to create file \"$out_fn\": $!\n");
   } else {
      print $out_fh $_;
   }
}
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Update: Removed chomp.
Update: Removed unneeded undef $out_fh;.