#! perl
use strict;
use warnings;
use Data::Dump;

my %hash;

while (<DATA>)
{
    push @{ $hash{$2} }, $1 if / ( ([^\s_]+) _ [^\s-]+ - \S+ ) /x;
}

print "\nData structure (HoA):\n";
dd \%hash;

print "\nOutput:\n";

for (sort keys %hash)
{
    my $array_ref = $hash{$_};
    print $_, ' ', scalar @$array_ref, ' ', join(' ', @$array_ref), "\n";
}

__DATA__
foo_1-a
foo_2-b
foo_3-b
foo_4-b
bar_1-a
bar_2-a
bar_3-b
bar_4-a
bar_5-b