Strange. I pasted you code into a file and it syntax checks clean:

C:\test>type junk.pl
#!/usr/bin/perl -w
# vanillaPrimes.pl

use strict;

my $divider; # Used for testing file.
open FILE, "<", "DATA1.txt" or die $!; # Open a file or return error.
open oFILE, "+>>", "OUT1.txt" or die $!; # Used to write answer.
my $numInput = <FILE>; # Getting the line from file.
chomp($numInput); # removing a newline, just in case.
my $half;
if($numInput % $divider == 0){
     $half = $numInput/2
} else {
     $half = ($numInput - 1)/2
}
foreach($divider..$half){
    if($numInput % $divider == 0){
        print oFILE "not";
    } else {
        print oFILE "prime";
    }
}
close(FILE);
close(oFILE);

C:\test>perl -c junk.pl
junk.pl syntax OK
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