$ perl -lE '
my ($haystack,$needle)=@ARGV;
my $i=0;
my @found;
while(-1 != (my $curidx = index $haystack, $needle, $i)) {
  push @found, $curidx;
  $i = $curidx+1
};
say "found at $_" for @found
' abcsdfabcasegabc abc
found at 0
found at 6
found at 13