use strict;
use warnings;

my @a = qw{ 0 1 5 7 9 5 3 };
my @b = qw{ a b c y e f z a a };

die qq{Element count mismatch\n}
   unless scalar @b == scalar @a + grep m{[yz]}, @b;

print qq{@a\n@b\n};

my $offA = $#a;
foreach my $offB ( reverse 0 .. $#b )
{
    if( $b[ $offB ] =~ m{[yz]} )
    {
        splice @a, $offA + 1, 0, q{I};
    }
    else
    {
        $offA --;
    }
}

print qq{@a\n};

##</code><code>##

0 1 5 7 9 5 3
a b c y e f z a a
0 1 5 I 7 9 I 5 3