Documentation for variable $_ assembled from the following types:

language documentation Variables

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(Variables) variable $_

$_ is the topic variable. It's the default parameter for blocks that do not have an explicit signature, so constructs like for @array { ... } and given $var { ... } bind the value or values of the variable to $_ by invoking the block.

for <a b c> { say $_ }  # sets $_ to 'a', 'b' and 'c' in turn
say $_ for <a b c>;     # same, even though it's not a block
given 'a'   { say $_ }  # sets $_ to 'a'
say $_ given 'a';       # same, even though it's not a block

CATCH blocks set $_ to the exception that was caught. The ~~ smartmatch operator sets $_ on the right-hand side expression to the value of the left-hand side.

Calling a method on $_ can be shortened by leaving off the variable name:

.say;                   # same as $_.say

m/regex/ and /regex/ regex matches and s/regex/subst/ substitutions work on $_:

say "Looking for strings with non-alphabetic characters...";
for <ab:c d$e fgh ij*> {
    .say if m/<-alpha>/;
}

# OUTPUT: «Looking for strings with non-alphabetic characters...
#          ab:c
#          d$e
#          ij*␤»