use warnings;
use strict;
use Data::Dump;  # debug

my @vgmap = (
	{ node => "node1", vg => "vg1", maj_num => 36 },
	{ node => "node2", vg => "vg1", maj_num => 36 },
	{ node => "node1", vg => "vg2", maj_num => 37 },
	{ node => "node2", vg => "vg2", maj_num => 37 },
	{ node => "node1", vg => "vg3", maj_num => 38 },
	{ node => "node2", vg => "vg3", maj_num => 40 },
);

my %vgmap;
for my $vgm (@vgmap) {
	push @{ $vgmap{ $vgm->{vg} }{ $vgm->{maj_num} } }, $vgm->{node};
}
dd \%vgmap;  # debug

for my $vg (sort keys %vgmap) {
	my @maj_nums = sort keys %{ $vgmap{$vg} };
	if ( @maj_nums != 1 ) {
		print "$vg: ", join( "; ",
			map { "$_ used on "
				. join ", ", @{ $vgmap{$vg}{$_}} } @maj_nums
			), "\n";
	}
}

__END__

{
  vg1 => { 36 => ["node1", "node2"] },
  vg2 => { 37 => ["node1", "node2"] },
  vg3 => { 38 => ["node1"], 40 => ["node2"] },
}
vg3: 38 used on node1; 40 used on node2