I tried to compile this code but without success (both, MiKTeX and TeXLive, generate similar logs).

Here's the code

mode=localfont;
input plain;

beginchar(65, 10pt#, 10pt#, 0);
  pickup pencircle scaled 0.5pt;
  draw (0,0) .. (5,10) .. (10,0) .. cycle;
endchar;

end.

On the cmd prompt (>) I typed:

mf test.mf

It waits a while then logged this:

This is METAFONT, Version 2.71828182 (MiKTeX 25.2) (preloaded base=mf 2025.3.10)  10 MAR 2025 20:18
\*\*test.mf
(test.mf (D:\\Program Files\\MiKTeX\\metafont/base\\plain.mf
Preloading the plain base, version 2.71: preliminaries,
basic constants and mathematical macros,
! Missing \`)' has been inserted.
\<to be read again\>
(future pen)
l.88 capsule_def(pensquare
) makepen(unitsquare shifted -(.5,.5));

I've gotten to the end of the macro parameter list.
You might want to delete some tokens before continuing.

! Missing `=' has been inserted. <to be read again>                     (future pen) capsule_def->def(SUFFIX0)=(EXPR1)                                  enddef <to be read again>                     ) l.88 capsule_def(pensquare)                             makepen(unitsquare shifted -(.5,.5)); The next thing in this `def' should have been `=', because I've already looked at the definition heading. But don't worry; I'll pretend that an equals sign was present. Everything from here to `enddef'
will be the replacement text of this macro.

! Extra tokens will be flushed.
\<to be read again\>
)
l.88 capsule_def(pensquare)
makepen(unitsquare shifted -(.5,.5));

I've just read as much of that statement as I could fathom,
so a semicolon should have been next. It's very puzzling...
but I'll try to get myself back together, by ignoring
everything up to the next \`;'. Please insert a semicolon
now in front of anything that you don't want me to delete.
(See Chapter 27 of The METAFONTbook for an example.)

! Missing \`)' has been inserted.
...

It continued with similar errors each time { ! Missing 'Some Character' }