#!perl
use strict;
use warnings;
my $File = "input.txt"; # this string wasn't quoted, so you must have not shown us the code you're using
open(FILE, "<", $File) or die qq(failed to open file "$File": $!); # you didn't check whether open failed
chomp(my @IPLIST = <FILE>); # this works as expected
close(FILE);

foreach my $test (@IPLIST) {
  print "Is $test an IP?\n";
}