If you know the number of lines, e.g if you get an array on standard input, or passed to a subroutine:

use strict; use warnings;
use feature qw/ say /;

chomp( my @lines = (<DATA>) );
my $num = @lines;
my $count = 1;
for my $line ( @lines ) {
    say "the last line is $line" if $num == $count++;
}

__DATA__
foo
bar
baz
qux
[download]

$ perl 1202892.pl
the last line is qux
[download]

If you don't know the number of lines, e.g. when reading a file that may be too large to slurp into memory all at once:

use strict; use warnings;
use feature qw/ say /;

# setting up the demo
use Path::Tiny;
my $file = Path::Tiny->tempfile;
$file->spew("$_\n") for ('a' .. 'm');
#######

my $last_line;
open my $fh, '<', $file or die "Cannot open file: $!";
while ( my $line = <$fh> ) {
    chomp $line;
    $last_line = $line;
}

say "the last line is $last_line";

__END__
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$ perl 1202892-2.pl
the last line is m
[download]

Hope this helps!