Are you looking for something like this?

#! perl -slw
use strict;

## Adjust to suit your requirements
my %types = (
   lower => [ 'a'..'z' ],
   upper => [ 'A'..'Z' ],
   number=> [ '0'..'9' ],
   char  => [ 'a'..'z', 'A'..'Z', '0'..'9' ],
);

print join ' ', map{ 
    my( $type, $n ) = $_ =~ m[(\w+) length = (\d+)];
    join'', map{ 
        $types{ $type }[ rand @{ $types{ $type } } ] 
    } 1 .. $n;
} @ARGV;

__END__
P:\test>413131 "char length = 4" "number length = 3" "lower length = 6
+"
tAwY 828 xkppno

[12:35:08.56] P:\test>413131 "char length = 1" "number length = 10" "u
+pper length = 2"
j 3625181636 OB
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