• $minlength+=int(rand($maxlength))
  should be
  $minlength+int(rand($maxlength-$minlength))

• You want a number in [0..$#vchars], but
  int(rand($#vchars))
  returns a number in [0..$#vchars-1]. You want
  int(rand(@vchars))
  The int() isn't needed here since array indexes are coerced into ints already.

• my $cnt=1; while ($cnt <= $count) { ... $cnt++; }
  would be cleaner as
  for (1..$count) { ... }

• Similarly for the inner loop.

• @vchars only needs to be initialized once, not $count times.

use strict;
use warnings;

my $count     = 10;
my $minlength = 3;   
my $maxlength = 6;

my @vchars = ('a'..'z', 'A'..'Z', 0..9);

for (1 .. $count) {
   my $data = '';

   for (1 .. $minlength+int(rand($maxlength-$minlength))) {
      $data .= $vchars[rand(@vchars)];
   }

   print("$data\n");
}
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