use strict;
use warnings;
use 5.010;

my $filename = "/etc/passwd";
my $size = -s $filename;
say $size;

stat function

Alternatively the stat built-in function of perl returns a 13-element list providing information about the status of the file. The 8th element (index 7) is the size of the file.

use strict;
use warnings;
use 5.010;

my $filename = "/etc/passwd";
my @stat = stat $filename;
say $stat[7];

Fetch list element on-the-fly

Of course you don't have to assign the return value of the stat function to an array. We can fetch the element on-the-fly by putting parentheses around the whole expression and then putting the element index after that in square bracket: (stat $filename)[7];

use strict;
use warnings;
use 5.010;

my $filename = "/etc/passwd";
my $size = (stat $filename)[7];
say $size;

We don't even need the $size variable for that, but we cannot simply write say (stat $filename)[7]; or print (stat $filename)[7];.

The reason is that in this case perl will think the parentheses are part of the say or print function and the [7] is an index on the return value of say or print.

We can solve this by either adding the real parentheses of the say/print functions, or by adding a + sign in-front of the parentheses:

use strict;
use warnings;
use 5.010;

my $filename = "/etc/passwd";
say ((stat $filename)[7]);
say +(stat $filename)[7];

Object Oriented

Probably the most readable of all the solutions is the one using the File::stat module. It provides a replacement for the stat function of perl that will return an object which has, among several others, a method called size that will return the size of the file.

Here too, the first version assigns the object to a variable called $stat, and the second method calls the size method on-the-fly, without the need of the extra variable.

use strict;
use warnings;
use 5.010;

my $filename = "/etc/passwd";

use File::stat;
my $stat = stat($filename);
say $stat->size;

say stat($filename)->size;