Your last description calls for reprocessing @extras until it is empty:

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
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I like how the for replaces the bare block for scoping.

You can also do it without the @new_extras variable by replacing the foreach loop/push combination with a map:

my @foo = qw/a b c d e f g/;
my $sub = 'h';
for (my @extras = @foo; @extras;) {
    @extras = map { /[dhij]/ ? $sub++ : () } @extras;
    push @foo, @extras;
}
print for @foo;
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