You're doing a substitution at every line.

A "one-liner" solution:

perl -i.bak -nle'
    BEGIN { print("IPADDR=", shift(@ARGV)) }
    print if !/^IPADDR=/;
' 1.1.1.1 dir/sys/network-scripts/ifcfg-eth0
[download]

A script solution:

print("IPADDR=$ip\n");
while (<$fh_in>) {
    print $fh_out $_ if !/^IPADDR=/;
}
[download]

If you wanted to use substitution (to maintain order), it would look like

my $found = 0;
while (<$fh_in>) {
    ++$found if s/^IPADDR=.*/IPADDR=$ip/;
    print $fh_out $_;
}

print("IPADDR=$ip\n") if !$found;
[download]