so, with trivial variants on method above:

#!C:/Perl/bin
use strict;            # no warnings because using uninit values below
use Data::Dumper::Simple;


use vars qw ( @nomatch $I1 $I2 $I3 $L1 $L2 @data $i $j );
while (<DATA>)
   {
      push @data,$_ ;
   }

{  
   while (@data) 
      {
      $L2 = pop @data;
      chomp $L2;
      #print "\$L2 is: $L2\n";
      $L1 = pop @data;
      chomp $L1;
      #print "\$L1 is: $L1\n";

      #find the data here

      if ( $L1 =~ /
            \d\d\d      # three digits
            \s+         # one or more whitespace
            NP          # exact string, NP
            \s+         # one or more whitespace
            U           # exact string, U
            \s+         # one or more whitespace
            Pu          # exact string, Pu
            /x          # end match, extended
         && $L2 =~ /
            (\d\d\d)    # three digits
            \s+         # one or more whitespace
            (\d\.\d{6}) # digit, period, six digits
            \s+         # one or more whitespace
            (\d{6})     # six digits
            \s+         # one or more whitespace
            (\d\.\d{6}) # digit, period, six digits
            /x )
        {
          my $n1 = $1; $I1 = $2; $I2=$3; $I3=$4;
          print "\n\tIn linepair ENDING with $n1, NP: $I1, U: $I2, Pu:
+ $I3\n";
        } 
        else
        {
          push @nomatch,"\n\tNo match on lines    $L1\n\t\t\t and $L2\
+n";
        }
      }
print "\n\n\t No Match pairs follow\n";
warn Dumper (@nomatch);
}
__DATA__
000    NP          U          Pu
001    1.270000    000001     3.141000
002    Lev         N          Pu
003    0.13        000001     3.277118
004    NP          U          Pu
005    1.000220    000002     3.098761
006    Yac         S          Yb
007    10.33000    000001     90000000
008    NP          U          Pu
009    2.130000    000140     5.797712
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