Question

Does calling map { function($_) } <FILEHANDLE>; load the entire file into memory when using perl?

Answer 1

Yes — or at least that’s how I interpret this outcome.

$ perl -e "map {0} <>" big_data_file
Out of memory!

$ perl -e "map {0} 1 .. 1000000000"
Out of memory!

One might wonder whether we run out of memory because Perl is trying to store the output of map. However, my understanding is that Perl is optimized to avoid that work whenever map is called in a void context. For a specific example, see the discussion in this question.

Perhaps a better example:

$ perl -e "sub nothing {}  map nothing(), <>" big_data_file
Out of memory!

Based on the comments, it appears that the question is motivated by a desire for a compact syntax when processing large data.

open(my $handle, '<', 'big_data_file') or die $!;

# An ordinary while loop to process a data file.
while (my $line = <$handle>){
    foo($line);
}

# Here Perl assigns each line to $_.
while (<$handle>){
    foo($_);
}

# And here we do the same thing on one line.
foo($_) while <$handle>;