unless statement in perl behaves just opposite to the if statement in perl. It executes the set of statements inside its body when the given condition is false.
unless(condition) {
statement(s);
}
The statements inside unless body would execute when the condition is false.
Example
#!/usr/local/bin/perl
printf "Enter any number:";
$num = <STDIN>;
unless($num>=100) {
printf "num is less than 100\n";
}
Output:
Enter any number:99 num is less than 100