Use hash keys to avoid lots of variables. Push the field names, values onto arrays and use join to create the statement

#!perl
use strict;

my $user_id = 1;

my $old = {
  password => 'password', forename  => 'forename',
  lastname => 'lastname', business => 'business',
  city     => 'city',     zip      => 'zip'
};

my $new = {
  password => 'password1',  forename  => 'forename',
  lastname => 'lastname1', business => 'business',
  city     => 'city1',     zip      => 'zip1'
};

my @fields;
my @values;
# compare old v new
for (sort keys %$new){
  if ($new->{$_} ne $old->{$_}){
    push @fields,"$_ = ?";
    push @values,$new->{$_};
  }
}

# skip if no change
if (@fields == 0){
  print "No update required\n";
} else {
  # build sql
  my $fields = join ',',@fields;
  my $stmt = "
   UPDATE table 
   SET $fields
   WHERE user_id = ?";
   # add id
   push @values,$user_id;
   
   # prepare and execute sql
   print "$stmt\n(@values)\n"; 
}
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