I don't have a windoze box. On Linux with 5.8.8 I do see the warn "Insecure dependency in kill while running with -T switch at /usr/lib/perl5/site_perl/5.8.8/WWW/Mechanize.pm line 2446." I do not see a crash, but valgrind (which is a utility that interprets machine code and checks all memory accesses carefully) says that there just after that warn, there are some reads of memory that was free()ed, which is a bug that could easily cause a crash.
On Linux with 5.16.0 that warn is not there and valgrind's report is clean. So you'll probably find it's fixed if you upgrade. (I remember this valgrind-complains-under-taint symptom coming up on perl5-porters, in fact, and it was fixed.)
I set my tool to look for the message and it was confused by the fact that either "warn;" can cause the message to appear. This causes the message under 5.8.8:
#!/usr/bin/env perl -T
my $ua = WWW::Mechanize->new();
my $url = shift;