use strict;
use warnings;

# init the "valid hash"
# in real program, probably K1 etc probably
# come from some configuration file

my %hash = map{$_ => 1}qw (K1 ASC GEB N1);

# I use a simple fixed regex to pick up first XXX_YYY
# in the line. If there are multiple of these things
# per line, then there are a couple of ways to handle
# that. I'd have to see some actual real data to make
# a recommendation. Note that \w includes the underscore,
# so I did something simple to exclude the _ character.

# To check if match worked, you can put whole regex
# line in the "if" clause, or just check "definedness"
# like I do below. If $tok is defined in the hash,
# then it is a "good" one. 

while (my $line = <DATA>)
{
   my ($tok,$value) = $line =~ m/([A-Za-z0-9]+)_([A-Za-z0-9]+)/;
   
   if ( defined $value and defined $hash{$tok})
   {
      print "Valid TOKEN/VALUE pairs: $tok $value\n";
   }
}   

=prints
Valid TOKEN/VALUE pairs: K1 XXX
Valid TOKEN/VALUE pairs: GEB ZZZ
=cut

__DATA__
 32  K1_XXX bbb  _xxx  ASC_ccc
STUFF_YYY;
   M1_bbb;
GEB2_NO
GEB_ZZZ