Re^5: looking for a good idiom: return this if this is true

$_ is set by very few things:

The implicit setting for a while() loop reading from a filehandle:

while (<HANDLE>) { ... } # same as ...
while (defined($_ = <HANDLE>)) { ... }
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The implicit variable of a foreach loop:

foreach (@list) { ... } # same as foreach $_ (@list) { ... }
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The implicit variable of a grep/map block (added after prodding, sorry):

$count = grep { BLOCK } LIST;
@result = grep { BLOCK } LIST;
@result = map { BLOCK } LIST;
map { BLOCK } LIST; # invalidates your warranty
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I think that's it. I don't recall anything else that can set the value. Lots of operations can alter the value, like chomp or s///.

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