Here's your code with corrections.

open FILE, '< /path/to/storedata.txt' or die $!;
open OUTP, '> /path/to/exporteddata.txt' or die $!;
my $StoreNum = (split ' ', <FILE>, 2)[0];
while (<FILE>){
    chomp;
    print OUTP $_, ' ', $StoreNum, $/ or die $!;
}
close OUTP or die $!;
close FILE or die $!;
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After Compline,
Zaxo