ENTER / LEAVE & FIRST / LAST

Block-entry/exit phasers and loop first/last-iteration phasers.

Some phasers fire around blocks and loop iterations. ENTER/LEAVE run each time a block is entered and left; FIRST/LAST run on the first and last iteration of a loop.

ENTER and LEAVE #

ENTER runs on entering the block, LEAVE on leaving it — including via an exception or an early return, which makes LEAVE ideal for cleanup.

{
    ENTER say "enter";
    say "body";
    LEAVE say "leave";
}

enter
body
leave

FIRST and LAST #

Inside a loop, FIRST runs only on the first iteration and LAST only on the last, while the body runs every time.

for 1..3 {
    FIRST say "first";
    say $_;
    LAST say "last";
}

first
1
2
3
last

Notes #

• LEAVE is guaranteed to run however the block is exited, so it is the structured way to release resources — the equivalent of try/finally elsewhere.
• The full loop set is FIRST, NEXT (after each iteration), and LAST; NEXT is handy for per-iteration bookkeeping.
• Phasers see the lexical scope they are written in, so they can read and update the surrounding block's variables.