open my $filea, "<", "Population.txt" or die "Cannot open: $!\n";

my @Data = <$filea>;

##</code><code>##

# ...

foreach my $line (@Data) {
    # do something with $line
}

##</code><code>##

open my $filea, "<", "Population.txt" or die "Cannot open: $!\n";

while(<$filea>) {
    # do something with $_
}