You have the syntax of the for loop already, so I'm going to guess the problem is printing the array outside of the subroutine in which it is populated - correct? Define the array at the main level, and pass a reference to it to the subroutine. Then, any changes made to the array in the subroutine will be made to the original. See perlreftut

use strict;
use warnings;

my (@genome_data_g);
upstream(\@genome_data_g);

foreach $datapoint (@genome_data_g) {
    print "$datapoint\n";
}

sub upstream {
    my $array_ref = shift;
    $$array_ref[1] = 'some value';
}
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Update: removed & from subroutine call (line 5).