How about a simple grep:

my $str_a = 'Foo';                                # Can be any length
for my $str_b (grep /^$str_a/, @list ) {                         # Can
+ be any number of items
    expensive_function($str_a , $str_b);
}
[download]

Will call expensive_function only on items that start with 'Foo' (Which can be limited to one character if thats what you Really want)

Update: This is the same as delirium's solution, above.