I think you meant:

...
my $lineNum = 1;
while ( my $hr = $csv->getline_hr(*FILE) ) {
    my $sku = $hr->{ 'sku' };
    print "$sku\n";
    $lineNum++;
}
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The getline_hr() method - just as the underlying getline() - expects an IO object (Perl file handle), not a string.

To parse lines directly from strings, you'd say $csv->parse($line) — but there's no parse_hr() method  (you don't need it for what you're doing, as the above should work fine).

(When using $csv->getline_hr($line), I do get the exact same error.)