$ perl -Mstrict -Mwarnings -E '
    my (%x, %y) = (a => 1, b => 2, c => 3, d => 1, e => 2, f => 1);
    push @{$y{$x{$_}}}, $_ for keys %x;
    say "$_: ", join ", ", @{$y{$_}} for keys %y;
'
2: e, b
1: d, a, f
3: c