hackish solution:

sub print_variable {
    local *print_variable = $_[0];
    my $name = $print_variable;
    for (keys %::){
        next if /^print_variable$/;
        print "$_ = $name\n" 
            if *{$::{$_}}{SCALAR} eq  *{$::{print_variable}}{SCALAR};
    }
}
$firstname = "Edward";
$var = 1;
print_variable( *firstname );
print_variable( *var );
__END__
firstname = Edward
var = 1
[download]

Doesn't work with my variables, only with symbol table entries. Also, if you alias a typeglob, you get them all:

$firstname = "Edward";
*surname = *firstname;
print_variable( *firstname );
__END__
firstname = Edward
surname = Edward
[download]

update: fixed b0rken code...

_($_=" "x(1<<5)."?\n".q·/)Oo.  G°\        /
                              /\_¯/(q    /
----------------------------  \__(m.====·.(_("always off the crowd"))."·
");sub _{s./.($e="'Itrs `mnsgdq Gdbj O`qkdq")=~y/"-y/#-z/;$e.e && print}