use strict;
use warnings;

$x = 0;  # syntax error: $x has not been declared
my $y = 1;  # ok, $y is "in scope" for remainder of file

{
    my $y = 2;  # a different $y, scope limited to this block
    print $y, $/;  # you cannot "see" the "outer" instance of $y
    sub foo { $y--; print "the inner \$y is now $y\n"; }
    foo();
}

print $y, $/;  #  this is the "outer" $y

my $x = 3;   # ok: $x is in scope for remainder of file
my $y = 4;   # warning: "my" declaration masks earlier declaration in same scope

print $y,$/;
foo();