>perl -wMstrict -le
"my $s = 'x foo1 foo2 x foo3 x yfoo9 foo4 foo5 foo9y x foo6 foo7 x';
 print qq{'$s' \n};
 ;;
 my $pat = qr{ \b foo \d \b }xms;
 ;;
 for my $n (0 .. 4) {
   (my $t = $s) =~ s{ \A (?: .*? $pat){$n} \K | $pat }''xmsg;
   print qq{'$t'};
   }
"
'x foo1 foo2 x foo3 x yfoo9 foo4 foo5 foo9y x foo6 foo7 x'

'x   x  x yfoo9   foo9y x   x'
'x foo1  x  x yfoo9   foo9y x   x'
'x foo1 foo2 x  x yfoo9   foo9y x   x'
'x foo1 foo2 x foo3 x yfoo9   foo9y x   x'
'x foo1 foo2 x foo3 x yfoo9 foo4  foo9y x   x'