You could create a static table of "neighbors" for each allowable password character. (I interpreted "sequential character" to mean any character that touches the current char in any direction):

$neighbors = {
   a => { 
     q => 1,
     w => 1,
     s => 1,
     z => 1,
   },
   s => {
     a => 1,
     w => 1,
     e => 1,
     d => 1,
     x => 1,
     z => 1,
   },
 
   etc.
}
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This hash would be specific to the type of keyboard being used.

You could then traverse, character-by-character, through the password to see if three successive finds occur in the neighbors hash for each letter traversed.

#!/usr/bin/perl

use strict;
use warnings;

my $neighbors = {
   a => {
     q => 1,
     w => 1,
     s => 1,
     z => 1,
   },
   s => {
     a => 1,
     w => 1,
     e => 1,
     d => 1,
     x => 1,
     z => 1,
   },
};


my $password = "asasas";
my $counter = 1;

my $lastchar = "";
for my $char (split(//, $password)) {
  print $char, "\n";
  $counter++ if $neighbors->{$lastchar}->{$char};
  $lastchar = $char;
  last if $counter == 3;
}

if ($counter == 3) {
  print "Invalid password";
  # make them enter a new password...
}
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