Is there a way to count the size of what's in the STDIN (all lines and not just one) other than saving it to a variable and using length()?

If you mean without reading until EOF, then no.

If STDIN is connected to the terminal, then there is no way to predetermine how much the user may type before hitting ^Z/^D.

If STDIN is connected to a pipe, there is no way to guess how much output the program on the other end might output. It might produce nothing, or never stop producing.

If you just mean without storing all of the content, but you are happy to read it all, then you can accumulate the lengths of all the lines without storing them:

my $count = 0;
$count += length while <STDIN>;
print $count;
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