You still need to loop but its easier written with pos and looping over a regex with /g modifier.

Something like:

 print pos while /substr/g ¹

edit

Though you might need to subtract the length of the match if you need the start position.

Of course TIMTOWTDI. :)

update

¹) corrected for to while

test:

  DB<107> $_='abcd'x4
 => "abcdabcdabcdabcd"

  DB<108> print pos()-2 while m/ab/g 
04812
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