Question

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How can I check if I have a Perl module before using it?

I’d like to eval a use statement similar to this: eval {use $foo;} but am having trouble with the correct syntax. I’ve tried various combinations of string interpolation, but the eval always succeeds even for a module that does not exist. Can someone give me hand with this one?

Answer 1

The idiom you are looking for is:

eval "use $module; 1" or ... ;

eval "use $module; 1" or warn "$module is not available: $@";
eval "use $module; 1" or die "This script requires $module";
eval "use $module; 1" or $module_available = 0;

eval “always succeeds” in the sense that program execution will continue no matter what code is evaluated — that’s really the whole point of an eval statement. If the evaluated string contained errors — run time or compile time — the return value of the eval call is undef and the special variable $@ will be set with an appropriate error message.

The use statement, even when successful, does not return a value. That’s why this idiom includes the “; 1” at the end.