use strict;
use warnings;

sub is_pw_valid {
    local $_ = $_[0];

    return
          !(/[a-z]/ && /[A-Z]/)                     ? length >= 20 
        : !/\d/                                     ? length >= 16
        : !/[\x21-\x2F\x3A-\x40\x5B-\x60\x7B-\x7E]/ ? length >= 12
        :                                             length >=  8
    ;
}
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UPDATE: Corrected error in regex for special characters and added (below) a less terse version of the same logic

sub is_pw_valid {
    my $password = $_[0];
    my $MixedCase = qr/(:? [a-z].*[A-Z]) | (:? [A-Z].*[a-z] )/x;
    my $Digit     = qr/\d/;
    my $special   = qr/[\x21-\x2F\x3A-\x40\x5B-\x60\x7B-\x7E]/;
    my $min_length;

    if    ($password !~ $MixedCase) {$min_length = 20}
    elsif ($password !~ $Digit)     {$min_length = 16}
    elsif ($password !~ $special)   {$min_length = 12}
    else                            {$min_length =  8}                
+      

   my $is_OK = ( length($password) >= $min_length );
   return $is_OK;
}
[download]