use Data::Dumper;
my $data = <<EOT;
Row 1
   label1 : item1
   label2 : item1
Row 2
   label1 : item2
   label2 : item2
EOT

push @{$hash{$1}}, $2 while $data =~ /(\S+)\s*:\s*(\S+)/g;

print Dumper(\%hash);