That ensures that the value of $1 from a successful match does not leak outside the block, but it doesn't protect you from $1 from an earlier successful match leaking into the block:

#!/usr/local/bin/perl -w

use strict;

$_ = 'perlmonks.org';

for my $re (qw/monks minks/) {
  /(perl)/;

  print "$1\n";

  {
    # new block for regex
    /($re)/;
    print "$1\n";
  }
}
__END__
perl
monks
perl
perl
[download]

It's generally necessary to check whether a regex succeeded before using any of the regex special variables.