Programming Perl, 4th Edition

Bare Blocks as Loops

A BLOCK by itself (labelled or not) is semantically equivalent to a loop that executes once. Thus, you can use last to leave the block or redo to restart the block.^[79] Note that this is not true of the blocks in eval {}, sub {}, or, much to everyone’s surprise, do {}. These three are not loop blocks because they’re not BLOCKs by themselves; the keyword in front makes them mere terms in an expression that just happen to include a code block. Since they’re not loop blocks, they cannot be given a label to apply loop controls to. Loop controls may only be used on true loops, just as a return may only be used within a subroutine (well, or an eval).

Loop controls don’t work in an if or unless, either, since those aren’t loops. But you can always introduce an extra set of braces to give yourself a bare block, which does count as a loop:

if (/pattern/) {{
    last if /alpha/;
    last if /beta/;
    last if /gamma/;
    # do something here only if still in if()
}}

Here’s how a block can be used to let loop-control operators work with a do {} construct. To next or redo a do, put a bare block inside:

do {{
    next if $x == $y;
    # do something here
}} until $x++ > $z;

For last, you have to be more elaborate:

{
    do {
        last if $x = $y ** 2;
        # do something here
    } while $x++ <= $z;
}

And if you want both loop controls available, you’ll have to put a label on those blocks so you can tell them apart:

DO_LAST: { do { DO_NEXT: { next DO_NEXT if $x == $y; last DO_LAST if $x = $y ** 2; # do something here } ...