my $i=100;
# Some code here...
...

sub_1 {
my $i; # This $i is undefined.  It's not the same $i that was defined outside the sub.
# Code that uses $i
...
}

# $i now = 100, as before, since we're not in the sub.

sub_2 {
my $i = 4; # This $i = 4.
print "$i\n"; # Prints 4.
}

# Once again, $i = 100.