You could use substr for this:

use strict;
use warnings;

my $string = 'That string is funny!';

if ( substr( $string, 2, 1 ) eq 'a' ) {
    print 'The third character in $string is an "a".';
}
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But if you're set on a regex, the following will match a string whose third character is "a":

$string =~ /^..a/
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