It is usually considered bad form to modify $_ during a map{} operation. People expect map{} to create a new list from an existing list, without modifying the existing list.

The expression can be written out clearer, more accurately, and using less intermediate memory by using the following code instead:

my %hash;
for (@array) {
    if (/^([^,]+)(?:,(.*))?$/) {
        $hash{$1} = $2;
    }
}
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This also has the side effect of handling the case where the input takes the form "1009," or "1009". For "1009," the key will map to an empty string. For "1009" the key will map to the undefined value.

The regular expression removes the need for chomp() as the $ at the end of the regular expression will not swallow the "\n".

If efficiency is the true goal, the following code has the exact same effect:

my %hash;
/^([^,]+)(?:,(.*))?$/s && ($hash{$1} = $2) for @array;
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If a few CPU ticks can be spared, the former is far more preferable to the latter.

Just ensure that there is something after the comma... even if it is a space.
If you have no control over the data then perhaps try something like this:
my %hash = map { s/,\n/, \n/; chomp; split /,/ } @array;