splice() is fun. (Pls. to pardon ugly.)

#!/usr/bin/perl

@a=("a","b","c","b","a");
$targ="b";
$ins="d";
THINGY: foreach(@a) {
    $_ =~ $targ && do {
        splice(@a,$i,1,($_, $ins));
        last THINGY
    };
    $i++;
};
print join ", ", @a;
print "\n"
[download]

Output:

a, b, d, c, b, a
[download]

It's not entirely cool; it does take an iterator, which I accept is fairly cheesy, but it does the job in a single foreach() that probably won't even make it to the end of the array.