This will ensure that the string is exactly six characters, only consists of alpha-num, and matches the 5th char:

use warnings;
use strict;

my $str = 'xpy7pG';
my $char = 'p';

if ($str =~ /^\w{4}$char\w$/){
    print "matched\n";
}
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update: I lied. The above will also match an underscore (as \w matches [a-zA-Z0-9_]. If you don't want that, you can use this:

/^[0-9A-Za-z]{4}$char[0-9A-Za-z]$/
[download]