Yes, a file with a gz extension is still a file^*.

Here is an example that shows two ways to get the size of a file: using the file test -s (see -X in perlfunc) and the function stat. Note that both of these may not be 100% portable, as per the docs.

use strict;
use warnings;

my $file = 'find_my_size.tar.gz';

my $size = -s $file;
print "size of $file by -s is $size\n";

my @filedata = stat $file;
print "size of $file by stat is $filedata[7]\n";
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^*Unless it's a directory that ends in .gz, etc