# yes, I realise there is NO error checking :)
# assume $tree contains data structure shown above
# assume input = "/docs/howtos/email/"

my $string = join('}->{',split(/\//,$input));
$string =~ s/^.(.+)/$1}/;
$string = '$tree'.$string."->{'index.html'}->{size};";

# string now contains: "$tree->{docs}->{howtos}->{email}->{'index.html'}->{size};"

# constructed reference
print eval $string;

# explicit reference
print $tree->{docs}->{howtos}->{email}->{'index.html'}->{size};