use strict;
use warnings;

my @x = ("aaBxyz", "..Bxys", "bbxyzzy", "xxAx");

# for each element in the array @x, 
# pass that element to the array @result if
# if the 3rd character is either A or B
# followed by at least one character.

my @result = grep {/^..(A|B)./}@x;

print "$_\n" for @result;

=prints
aaBxyz
..Bxys
xxAx
=cut