Just to add my $0.02. I didn't use a hash at all, I used grep. 
use strict;

my @Group1 = qw( H0 K0 PA PB PC PD PE PF PG PH );
my @Group2 = qw( PX PY PZ P1 P2 P3 P4 P5 P6 P7 );

my @G1_out;
my @G2_out;

while (my $input = <DATA>) {
    chomp ($input);
    my $prefix = substr($input,0,2);

    # NB: grep is slow in this case. evil. beware.
    push (@G1_out, $input) if grep($_ eq $prefix, @Group1);
    push (@G2_out, $input) if grep($_ eq $prefix, @Group2);
}

print "G1\n";
print "$_\n" foreach @G1_out;
print "\nG2\n";
print "$_\n" foreach @G2_out;

__DATA__
A1 # invalid
K0 # valid G1
B4 # invalid
PY # valid G2

[download]
Gives: 
G1
K0 # valid G1

G2
PY # valid G2

[download]
Update: I guess i should've mentioned that i knew it was slower. The bonus I saw is that it doesn't require a hash per group, which i think is a good thing. I guess my priorities lie elsewhere =) My bad.

--
Rock is dead. Long live paper and scissors!

In reply to Re: Efficient Grouping by LTjake
in thread Efficient Grouping by meetraz

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