# your number 1 uses "if (not ...)"
# which is equivalent to "unless (...)"

# 4
unless (exists $sheets{$machine_sheetname}) {
    $sheets{$machine_sheetname} = 1;
}

# but I'm not really sure why you're checking for existence at all.
# just incrementing the value is the simplest approach.

# 5
$sheets{$machine_sheetname}++;