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when
Combined from primary sources listed below.

In Control flow§

See primary documentation in context for when.

The when block is similar to an if block and either or both can be used in an outer block; they also both have a "statement modifier" form. But there is a difference in how following code in the same, outer block is handled: When the when block is executed, control is passed to the enclosing block and following statements are ignored; but when the if block is executed, following statements are executed. [1] The following examples should illustrate the if or when block's default behavior assuming no special exit or other side effect statements are included in the if or when blocks:

{ if X {...} # if X is true in Boolean context, block is executed  # following statements are executed regardless } { when X {...} # if X is true in Boolean context, block is executed  # and control passes to the outer block  # following statements are NOT executed }

Should the if and when blocks above appear at file scope, following statements would be executed in each case.

There is one other feature when has that if doesn't: the when's Boolean context test defaults to $_ ~~ while the if's does not. That has an effect on how one uses the X in the when block without a value for $_ (it's Any in that case and Any smartmatches on True: Any ~~ True yields True). Consider the following:

{ my $a = 1; my $b = True; when $a { say 'a' }; # no output  when so $a { say 'a' } # a ("so $a" 'so' coerces $a to Boolean context True  # which matches with Any)  when $b { say 'b' }; # no output (this statement won't be run) }

Finally, when's statement modifier form does not affect execution of following statements either inside or outside of another block:

say "foo" when X; # if X is true statement is executed  # following statements are not affected

Since a successful match will exit the block, the behavior of this piece of code:

$_ = True; my $a; { $a = do when .so { "foo" } }; say $a; # OUTPUT: «(Any)␤»

is explained since the do block is abandoned before any value is stored or processed. However, in this case:

$_ = False; my $a; { $a = do when .so { "foo" } }; say $a; # OUTPUT: «False␤»

the block is not abandoned since the comparison is false, so $a will actually get a value.