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use Getopt::Long (); 

...

print("\$opt_float = ", defined($opt_float) ? $opt_float : "[undef]", 
+"\n");
print("\@ARGV      = ", join(' ', @ARGV), "\n");

>perl 551404.pl -float 1.3
$opt_float = 1.3
...
>perl 551404.pl -float abc
Value "abc" invalid for option float (real number expected)
Bad args