my @list = ...;
my %list; @list{ map{ substr $_, 0, 1 } @list } = ();

my $str_a = 'Foo';                               
my $first_c = substr $str_a, 0, 1;
for my $str_b ( @list ) {                         
    next if exists $list{ $first_char };  
    expensive_function($str_a , $str_b);
}