In your first example, perl is parsing $i as being a filehandle where output should be directed, which then makes the ++ operator the output which should be printed there. But print ++ for 1 is not valid, producing the syntax error you received:

$ perl -e 'print ++ for 1'
syntax error at -e line 1, near "++ for "
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In the second example, $i++ is unambiguous, because you can't increment a filehandle.

The parser must often guess which interpretation of the syntax is intended.

An isolated ++ could be a postincrement, a preincrement or a typo.

The parser is only playing safe by insisting that there is no gap.

G'day rsFalse,

I don't know what your question is. You got a syntax error; you fixed the syntax; the error went away.

See "perlop: Auto-increment and Auto-decrement". It shows a couple of examples of whitespace between the operator and operand. These are followed by:

"Perl will not guarantee what the result of the above statements is."

You should also use these pragmata:

• strict: which would have told you that $i had not been declared.
• warnings: which would have told you that $i was uninitialised.

If you had wanted to operate on the value(s) from for, that may have alerted you to the fact that $i was the wrong variable and you should have been using $_. See "perlsyn -- Perl Syntax".

Thanks for answers.