use strict;
my $str = '
<div class="tutu">
Blahblah.
<div class="whatever">
Blahblah0.
<div class="tata">
Blahblah1.
</div>
Blahblah2.
</div>
Blahblah3.
</div>
';
my $regex = qr/<div class="whatever">(.*?)(<div[^>]*>((?>(?:(?!<\/?div)[^\n])+|\n|(?2))*)<\/div>)(.*?)<\/div>/musp;
my $subst = '\\1\\2\\4';
my $result = $str =~ s/$regex/$subst/rg;
print "The result of the substitution is' $result\n";
Please keep in mind that these code samples are automatically generated and are not guaranteed to work. If you find any syntax errors, feel free to submit a bug report. For a full regex reference for Perl, please visit: http://perldoc.perl.org/perlre.html