use warnings;
use strict;

my @a = qw(0 1 5 7 9 5 3);
my @b = qw(a b c y e f z a a);
my @new;

while (@a && @b) {
    my $nextB = shift @b;

    if ($nextB =~ /[yz]/) {
        push @new, 'I';
    } else {
        push @new, shift @a;
    }
}

print "@new";

##</code><code>##

0 1 5 I 7 9 I 5 3

##</code><code>##

use warnings;
use strict;

my @a = qw(0 1 5 7 9 5 3);
my @b = qw(a b c y e f z a a);
my @new;
my $scanA = 0;

for my $nextB (@b) {
    if ($nextB =~ /[yz]/) {
        push @new, 'I';
    } else {
        last if $scanA >= @a;
        push @new, $a[$scanA++];
    }
}

print "@new";