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if ( $] >= 5.006 ) {
   print "This is good, your Perl is version $]\n";
...
else {
   warn "Your Perl is only version $]: Bad!";
}

require 5.006_001;  # check at run-time, at this point in code.
use 5.006_001;      # check at compile-time

eval {
   require 5.006_001;
...
   warn "Your Perl is too old."
   system ('n:\\install\\upgrade-perl.exe'); # imaginary
}

# import a version of a module at compile time, die on failure
use Module 1.02; # dies unless Module is at least v1.02
...
   warn "Module is not available";
   # maybe install it here
}

if (-f 'c:\\program files\\some app\\app.exe') {
   print "APP is installed, good.\n";
...
else {
   warn "APP not installed!";
}