There is a difference between your two loops.

for (my $i = 1; $i <= 9; $i++) {
  s/\s/;/;
}
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In this loop, the variable $i goes from 1 to 9 and each time round the loop the variable $_ has a substitution applied to it.

for (1..9) {
  s/\s/;/;
}
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In this loop, you don't have $i so $_ is used as the iterator variable. So when you apply the substitution to $_ you are applying it to the numbers 1 to 9.

I think that you were looking for this:

for my $i (1..9) {
  s/\s/;/;
}
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which does the same thing as your first loop.

I have a nagging feeling that this may be an XY Question and that perhaps you should explain what you're really trying to do.

In the first case the loop iterates 9 times and the substitution operates on $_, which is not the loop variable because $i is supplied.

In the second case the loop iterates 9 times and the substitutions operates on $_ which is the loop variable and takes the values 1 - 9 on successive passes through the loop (no loop variable supplied).

The behaviour of the second form can be made the same as the first by providing a loop variable:

for my $i (1..9) {s/\s/;/;}
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