The backslash is taking a reference to %opt (so getopts recieves a hashref which it populates). It's no different than

my %opt;
@ARGV > 0 and getopts( 'a:', \%opt ) or die <<"USAGE";
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or

my $opt = {};
@ARGV > 0 and getopts( 'a:', $opt ) or die <<"USAGE";
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Just looks a little funy. And I usually use Getopt::Long, but that's just me (actually recently I use this more often, but that's not really going to help you . . . :).