The gist of this is that perl's parser depends on previously symbols that are already defined to figure out how to parse things.

The same goes for languages like C, but the difference is that in perl you can define symbols at compilation time as a result of running other code, e.g.:

    $ perl -wE 'BEGIN { eval "sub spew { say \@_ }" if time % 2 == 0 } spew "hello"'
    hello
    $ perl -wE 'BEGIN { eval "sub spew { say \@_ }" if time % 2 == 0 } spew "hello"'
    Unquoted string "spew" may clash with future reserved word at -e line 1.

Here the program can be syntactically correct or incorrect depending on the compilation time.