use warnings;
use strict;
use Data::Dumper;

my ( %hashTable, %groupedHash );

while (<DATA>) {
	my ( $key, $val ) = split;
	$hashTable{$key} = $val;
}

groupByCol2( \%hashTable, \%groupedHash );

print Dumper \%groupedHash;

sub groupByCol2 {
	my ( $hashTableRef, $groupedHashRef ) = @_;
	for my $key ( keys %$hashTableRef ) {
		push @{ $$groupedHashRef{ $$hashTableRef{$key} } }, $key;
	}
}

__DATA__
name1	group1
name2	group2
name3	group1
name4	group4
name5	group2
name6	group7
name7	group4
name8	group5