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