my %sheets;
[...]

# The next three methods are equivalent. Pick whichever you like most.

# 1
if (not exists $sheets{$machine_sheetname}) {
    $sheets{$machine_sheetname} = 1;    # "1" is just a dummy existence flag
}

# 2
$sheets{$machine_sheetname} = 1 unless $sheets{$machine_sheetname};

# 3
$sheets{$machine_sheetname} ||= 1;

##</code><code>##

keys %sheets;