This is a fun little gotcha with version strings. The best way to require perl5.6.0 is with require 5.006;. Observe:

% perl5005 -e 'require 5.6'
Perl 5.6 required--this is only version 5.00503, stopped at -e line 1.

% perl56 -e 'require 5.6'
Perl v5.600.0 required--this is only v5.6.0, stopped (did you mean v5.
+6.0?) at -e line 1.

% perl56 -e 'require 5.6.0'

% perl5005 -e 'require 5.6.0'
Can't locate 5.60 in @INC (@INC contains: /usr/lib/perl5/5.00503/i386-
+linux /usr/lib/perl5/5.00503 /usr/lib/perl5/site_perl/5.005/i386-linu
+x /usr/lib/perl5/site_perl/5.005 .) at -e line 1.

% perl5005 -e 'require 5.006'
Perl 5.006 required--this is only version 5.00503, stopped at -e line 
+1.

% perl56 -e 'require 5.006'

%
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5.6 doesn't work because perl5.6 interprets it as 5.600 instead of 5.006. 5.6.0 doesn't work because perl5.005 and earlier interpret it as a "5.60" and try to load 5.60.pm. 5.006 is the only form that works in old and new versions of Perl.