Assuming the keys of your input hash cannot contain the '|' character. Straight forward:

sub reversehash
{
    my $href = shift;

    my %h;

    for (keys %$href)
    {
        my $v = $href->{$_};

        if (exists $h{$v}) { $h{$v} .= "|$_"; }
        else { $h{$v} = $_; }
    }

    return \%h;
}
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To impress the chicks:

sub reversehash
{
    my %h;

    push @{$h{$_[0]->{$_}}}, $_ for keys %{$_[0]};
    $h{$_} = join '|', @{$h{$_}} for keys %h;

    return \%h;
}
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