which error?

> $user = 'first.last@somewhere.com';

that's fine. You most likely did use double quotes instead

$user = "first.last@somewhere.com";

which will try to interpolate the array @somewhere.

But this is just a guess since you didn't give us details

update

compare https://perlmaven.com/quoted-interpolated-and-escaped-strings-in-perl

If I use single quotes, I get the error "Can't call method "login" on an undefined value at line x." Here's the relevant portion of the code:

use Net::FTP;

my ($ftp, $host, $user, $pass, $dir, $fpath);

$host = 'ftp.my.com';
$user = 'last.first@somewhere.com';
$pass = 'mypass';
$dir = '/my/file_path/onftp';

$fpath = 'myfileonserver_xml.zip';

$ftp = Net::FTP->new($host, Debug => 0);
$ftp->login($user, $pass) || die $ftp->message;  ##this is line x##
$ftp->cwd($dir);
$ftp->put($fpath) || die $ftp->message;
$ftp->quit;
[download]

From the Net::FTP docs:

$ftp = Net::FTP->new("some.host.name", Debug => 0)
      or die "Cannot connect to some.host.name: $@";
[download]

Check for errors as shown in the docs. What does that say?

You are missing a semi-colon from the end of this line.

use Net::FTP
poj

You are getting the error on your first use of the $ftp object.

That means the construction of it in the line above failed, where you specified $ftp = Net::FTP->new($host, Debug => 0);

I can't see from here why it failed, you will have to debug if you can reach $host using some other method to ftp for instance.

Cheers, Sören

Créateur des bugs mobiles - let loose once, run everywhere.
(hooked on the Perl Programming language)