#! /usr/bin/perl
use strict;
use warnings;

# entweder
my $lines = qx{ wc -l test.txt };

# oder alles in Perl
my $lines = 0;
open my $fh, '<', 'test.txt' or die "test.txt: open failed: $!\n";
$lines++ while ( <$fh> );
close $fh;