use strict;
use warnings;

while (<DATA>) {
   my $index = index($_,'|');
   next unless $index;               # add error checking!!
   my $tag = substr($_,0,$index);

   expensive_function($tag,$_) if $tag =~ / /;
}

sub expensive_function {
   my ($tag,$line) = @_;
   my ($label,$number)  = split(' ',$tag,2);
   my ($digit) = $label =~ /_(\d+)$/;
   my @field = split(/\|/,$line);

   # now do something with all of this ...
}

__DATA__
START_1 123|
FILE 2222| XXXX| AAAA|
NEW |
END_1|