Control flow

if / unless / with

Full

Boolean branching with if/elsif/else and unless; definedness branching with with.

Conditionals choose a block to run. if branches on truth, unless on its negation, and with on definedness (rather than truth), topicalising the tested value.

if / elsif / else #

my $n = 7;
if $n > 5 {
    say "big";
}
elsif $n > 0 {
    say "small";
}
else {
    say "non-positive";
}

Output

big

unless #

unless runs its block when the condition is false. It takes no else — reach for if when you need both branches.

my $x = 0;
unless $x {
    say "falsy";
}
say "ok" if $x == 0;

Output

falsy
ok

with / without #

with runs when its argument is defined (not just true) and sets $_ to it; without is the complement. This is the clean way to handle "maybe a value".

my $v = 42;
with $v {
    say "defined: $_";
}
without Any {
    say "no value";
}

Output

defined: 42
no value

Notes #

All of these are expressions in statement position; for a value-returning choice inside an expression use the ternary ?? !!.
The postfix forms — say $x if $cond, ... unless ..., ... with ... — read well for one-line guards.
with 0 { } runs: 0 is defined even though it is false. That is the whole point of with versus if.