sub isleap {
    my ($year) = @_;
    return 1 if (( $year % 400 ) == 0 ); # 400's are leap
    return 0 if (( $year % 100 ) == 0 ); # Other centuries are not
    return 1 if (( $year % 4 ) == 0 ); # All other 4's are leap
    return 0; # Everything else is not
}

##</code><code>##

not $year % 4 xor $year % 100 xor $year % 400;

##</code><code>##

print $_,  
    ((not $_ % 4 xor $_ % 100 xor $_ % 400) 
    ? ' is ' 
    : ' is not ', 
    'a leap year'), $/ 
    for 1996, 1997, 1998, 1999, 2000, 2004, 2100;

1996  is  a leap year
1997  is not  a leap year
1998  is not  a leap year
1999  is not  a leap year
2000  is  a leap year
2004  is  a leap year
2100  is not  a leap year