...the thing doesn't have to be recursive

The issue is that while running your code, perl internally builds up a call stack of impressive depth (218255 frames here — my attempt segfaulted at $count being around 43650 (with the default ulimit -s setting of 8192kB)), as shown by the backtrace based on the core dump:

$ ./665040.pl

Count, Last value:
Testing: 43650 -> 43650
Segmentation fault (core dumped)


$ gdb /usr/local/perl/5.10.0/bin/perl core
(...)
Core was generated by `/usr/local/perl/5.10.0/bin/perl ./665040.pl'.
Program terminated with signal 11, Segmentation fault.
#0  0x080acd57 in Perl_sv_clear ()
(gdb) bt
#0  0x080acd57 in Perl_sv_clear ()
#1  0x080ad378 in Perl_sv_free2 ()
#2  0x0809915f in Perl_av_undef ()
#3  0x080ad129 in Perl_sv_clear ()
#4  0x080ad378 in Perl_sv_free2 ()
#5  0x080acd5f in Perl_sv_clear ()
#6  0x080ad378 in Perl_sv_free2 ()
(...)
(...)
#218242 0x0809915f in Perl_av_undef ()
#218243 0x080ad129 in Perl_sv_clear ()
#218244 0x080ad378 in Perl_sv_free2 ()
#218245 0x080acd5f in Perl_sv_clear ()
#218246 0x080ad378 in Perl_sv_free2 ()
#218247 0x0809915f in Perl_av_undef ()
#218248 0x080ad129 in Perl_sv_clear ()
#218249 0x080ad378 in Perl_sv_free2 ()
#218250 0x080c79e0 in Perl_leave_scope ()
#218251 0x080a13a8 in Perl_pp_unstack ()
#218252 0x080a0bb8 in Perl_runops_standard ()
#218253 0x0809f652 in perl_run ()
#218254 0x0805ee8f in main ()
(gdb)
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