my @foo = qw/a b c d e f g/;
my $sub = 'h';
for (my @extras = @foo; @extras; @extras = my @new_extras) {
    foreach my $x (@extras) {
	push @new_extras, $sub++ if $x =~ /[dhij]/;
    }
    push @foo, @new_extras;
}
print for @foo;
# Tacks on h when it hits the d, then
# i for the h, j for the i, and k for the j
# yielding abcdefghijk