$ perl -lE '
my ($haystack,$needle,$new)=@ARGV;
while(-1 != (my $curidx = rindex $haystack, $needle)) {
  substr $haystack, $curidx, length($needle), $new
};
say "new string: $haystack" 
' abcsdfabcasegabc abc foo
new string: foosdffooasegfoo