Three problems:

• "|" is a special character in regexps. It needs to be escaped.

• You will look for "MSG-HDR" and "|UB" on the same line. The following would match:

  MSG-HDR |UB
  [download]

  The next in the code below fixes this.

• $orderfound is not always cleared when it should be. The following would match:

  MSG-HDR
  bla
  |UB
  [download]

  Or maybe that's ok? If it's ok, keep the $orderfound = 0; where it was.

Fix:

my $orderfound = 0;

while (my $line = <>) {
   if ($line =~ /MSG-HDR/) {
      $orderfound = 1; 
      next;
   }
      
   if ($orderfound && $line =~ /\|UB/) {
      print "$line\n";
   } 

   $orderfound = 0;
}
[download]