$ perl -Mstrict -Mwarnings -l junk
13

$ cat junk

sub butter {
   local $_ = 'effect / 7ACV06';
   return $1 + junk($_) if s/(\d+)//;
}
sub junk {
   local $_ = $_[0];
   return $1 if s/(\d+)//;
}

print butter();

$