Unfortunately, I am not able to test this at the moment. But, this *should* work.

    use Image::Magick;

    my $rc;
    my $img = Image::Magick->new();

    $rc = $img->Read('file.png');
    die "$rc\n" if $rc;

    $rc = $img->Write('file.jpeg');
    die "$rc\n" if $rc;
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