It means that 'nmake' finds the makefile (that was generated by running perl Makefile.PL) to be unparseable. This happens because the generated makefile was actually created for 'dmake' to use - and both 'dmake' and 'nmake' have slightly different rules regarding what a valid makefile should look like.
The reason that the generated makefile is written for 'dmake' is that $Config{make} is set to 'dmake'. (You can check that by running perl -V:make.) If you really wanted to, you could use 'nmake' with strawberry perl by opening up C:/strawberry/perl/lib/Config_heavy.pl and changing the line make='dmake' to make='nmake'. That would then reset $Config{make} to 'nmake', and the generated makefiles would be written using the rules that 'nmake' understand.
But ... it's probably just simpler to use 'dmake'. There's no advantage to be gained by using 'nmake'.