I just wanted to make sure that I understood what was going on here...

(these are set up for a win32 shell, change your quotes to single ticks to run on UNIX...) 
perl -e "print eval qq/require DBI; DBI::SQL_VARCHAR/ || $@;"
>>> prints 'DBI::SQL_VARCHAR'

perl -e "print eval qq/require DBI; DBI::SQL_XVARCHAR/ || $@;"
>>> prints 'DBI::SQL_XVARCHAR'

perl -e "print eval qq/require DBI; DBI::SQL_VARCHAR()/ || $@;"
>>> prints '12'

perl -e "print eval qq/require DBI; DBI::SQL_XVARCHAR()/ || $@;"
>>> prints 'Can't locate auto/DBI/SQL_XVARCHA.al...'

perl -e "$it=q/SQL_VARCHAR/; print eval qq/require DBI; DBI::$it/ || $
+@;"
>>> prints 'DBI::SQL_VARCHAR'

perl -e "$it=q/SQL_VARCHAR/; print eval qq/require DBI; DBI::$it()/ ||
+ $@;"
>>> prints '12'

perl -e "$it=q/SQL_XVARCHAR/; print eval qq/require DBI; DBI::$it/ || 
+$@;"
>>> prints 'DBI::SQL_XVARCHAR'

perl -e "$it=q/SQL_XVARCHAR/; print eval qq/require DBI; DBI::$it()/ |
+| $@;"
>>> prints 'Can't locate auto/DBI/SQL_XVARCHA.al...'

[download]
Since I'm using an eval "" instead of an eval {}, it's a run-time evaluation (and by that I mean that it's parsed at runtime, whereas eval {} has the opportunity to parse some/all of your eval {} statement at compile time).

So, if eval "" finds something in there that doesn't look like a function call or a variable, it appears to treat it as a raw string. (The '()' at the end of the constant will make it look like a function)

I kinda expected it to complain about a bareword, or to actually go resolve the bareword as a constant, though. My only insight to this, is that eval() translates barewords into strings themselves, instead of being left as barewords. In other words, 
   eval "DBI::SQL_VARCHAR";
   # is the same as 
   $it='DBI::SQL_VARCHAR'; eval { $it };

[download]
Am I off-base here? Is that the way it works, or is something else going on here?

This seems like a case where DWIM didn't seem work for me... ;)

-Dave

In reply to runtime eval() and constants by dpmott

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