> perl -anF_ -e '$F[1] and chomp,push @{$h{$F[0]}},$_}{print "$_ ",scalar @{$h{$_}}," ",join " ", @{$h{$_}},"\n" for sort keys %h' test.txt
bar 5 bar_1-a bar_2-a bar_3-b bar_4-a bar_5-b
foo 4 foo_1-a foo_2-b foo_3-b foo_4-b