Nice solution! We ran into this issue doing a header for a grid of open ports on network systems (input generated by nmap). Since we knew the max length of our input (5 digits for tcp ports), we use this solution:

sub numerically { $a <=> $b };
my @portsorted = sort numerically keys %ports;
my @header;
for (@portsorted)
{
    my @portchars = split //, $_ . ' ' x (5 - length());
    for ( 0 .. 4 )
    {
        push @{$header[$_]}, $portchars[$_];
    }
}

for (@header)
{
    print "\t\t  ", join('|',@{$_}), "\n";
}
print "IPs\t\t  ", '_|' x scalar(@portsorted), "\n";
__END__
outputs this (tabs notwithstanding):
     2|8|1|1|4|4|9
     2|0|1|3|4|4|3
      | |1|9|3|5|8
      | | | | | | 
      | | | | | | 
IPs  _|_|_|_|_|_|_|
[download]

Obviously yours has more appeal as a general solution, since we don't have options for alignment, spacing, or so forth.