use strict;
use warnings;
use Data::Dumper;

my $data    = {};
my @keys;

while (<DATA>)
{
   chomp;
   if ($_ =~ /<([^>\/]*)>/) {
     push @keys, $1;
   } elsif ($_ =~ /<\/([^>]*)>/) {
     if ($keys[-1] eq $1) {
         pop @keys;
     } else {
         die "Invalid Nesting";
     }
   } else {
     $data->{ join("-", @keys) } .= $_;
   }
}

print Dumper($data);


__DATA__
<tag1>
<fast>
Slow down boy!!!
</fast>
<slow>
Increase your speed.
</slow>
</tag1>
<tag2>
<fast>
Slow down boy!! BIG HEAD!
</fast>
<slow>
Increase your speed.
</slow>
</tag2>