Each code block creates a new scope.

#!/usr/bin/perl --
use strict;
use warnings;
{
    my $one = 1;
    my $two = 1;
    {
      my $one = 3; ## new $one
      my $two = 4; ## new $two
      print "$one $two\n"; # old $one $two
    }

    if( time ){

      my $one = 5; ## new $one
      my $two = 6; ## new $two
      print "$one $two\n"; # old $one $two
    }

    print "$one $two\n"; # old $one $two
}
### NO $one/$two exist here
## print "$one $two\n";

__END__
3 4
5 6
1 1
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See Coping with Scoping