If your code is cut-n-pasted, part of your problem lies in the snippet below, where you re-open the file you plan on printing to, but as read-only.

open(ALIVEIP, ">aliveip.list");
close(ALIVEIP);
open(ALIVEIP, "aliveip.list");
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Also, do you really need to chomp when reading from <PINGALLIP>? If you want each message printed on a line by itself, you can just use...

while ( my $line = <PINGALLIP>) {
   print ALIVEIP $line if $line =~ /is alive/;
}
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    --k.