Personally, if I'm creating a subroutine whose purpose is to create a specific sort of file header, I would rather just have the subroutine return a single string value that is the file header, so that the caller can use the return value and does not need to pass around a file handle -- e.g.:

open( OUTPUT, ">", "some.file" ) or die "$!";
print OUTPUT rm3dHeader();
print OUTPUT rm3dData();
close OUTPUT;

sub rm3dHeader
{
    return "START_OF_FILE\n\nDATEFORMAT=YYYYMMDD\nDECIMALSEPARATOR=.\n
+";
}

sub rm3dData
{
    return "this is the data\n";
}
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This would provide some flexibility that might be handy, and generally helps to keep the "make_header" logic independent from the "create/write/close file" logic.