#!/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"