use Data::Dumper;

my %hash = (
    a => 'f',
    b => 'g',
    c => 'f',
    d => 'h',
    e => 'f',
);

my %dups;
while (my ($e, $g) = each %hash) {
    push @{$dups{$g}}, $e
}

while (my ($g, $e) = each %dups) {
    say "$g is duplicated in @$e"
      if @$e > 1;
}

print Dumper \%dups;