This isn't related to being able to transform for into while, and appears to be a deliberate choice. The parser injects a constant "1" if either while() or the middle part of for(;;) are empty: perly.y:

/* Loops: while, until, for, and a bare block */
loop    :       label WHILE '(' remember texpr ')' mintro mblock cont
                        { PL_copline = (line_t)$2;
                            $$ = block_end($4,
                                   newSTATEOP(0, $1,
                                     newWHILEOP(0, 1, (LOOP*)Nullop,
                                                $2, $5, $8, $9, $7)));
+ }
...
        |       label FOR '(' remember mnexpr ';' texpr ';' mintro mne
+xpr ')'
                    mblock
                        /* basically fake up an initialize-while lines
+eq */
                        { OP *forop;

                          PL_copline = (line_t)$2;
                          forop = newSTATEOP(0, $1,
                                            newWHILEOP(0, 1, (LOOP*)Nu
+llop,
                                                $2, scalar($7),
                                                $12, $10, $9));
                          if ($5) {
                                forop = append_elem(OP_LINESEQ,
                                        newSTATEOP(0, ($1?savepv($1):N
+ullch),
                                                   $5),
                                        forop);
                          }

                          $$ = block_end($4, forop); }
...
/* Boolean expression */
texpr   :       /* NULL means true */
                        { (void)scan_num("1", &yylval); $$ = yylval.op
+val; }
        |       expr
        ;
[download]

("remember" starts a lexical scope, but doesn't use any tokens.)

AIUI, while () would be illegal if it had said

loop    :       label WHILE '(' remember expr ')' mintro mblock cont
[download]

instead.

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