gdluce:

I don't know the semantics of use very well (especially under CGI!), but if it acts like the normal unix PATH, then I'd guess that you have "." in your path and that display.pl is in the same directory that b.pl is in. Then when you're in the directory holding c.pl, when you execute it, display.pl can't be found. I'll leave it to you to research and/or verify.

More information might yield a better theory.

...roboticus

When your only tool is a hammer, all problems look like your thumb.

Update: added emphasized section.