It is fairly easy to translate your system call to pure perl:

use strict;
use warnings;

open my $dev, "<", "/proc/net/dev" or die $!;
my ($eth0) = grep {/eth0/} <$dev>;
close $dev;

my $k = (split /\s+/,$eth0)[1];
$k =~ s/eth0://g;
print "$k\n";
[download]

Outputs:

190722498
[download]