The obvious guess as to why grep is faster is that it's implemented in C. Any Perl built-in is going to be faster than the equivalent Perl code because it's compiled down to native machine code (and probably better optimized, to boot). It's the same reason why Perl's built-in lexical sort is faster than giving an explicit comparison routine (and why you're often better off munging the data before-hand so that you can use the built-in lexical sort; update: sometimes known as the Guttman Rosler Transform).

That said, given your example use of while vs. grep and assuming you're interested in the first or only match (as in the while example), I would like to recommend the oft-overlooked List::Util function first. It's implemented in C, so it should be as fast as grep (or nearly so), and has the advantage of stopping when a match is found. This could potentially save a lot of file IO in your example. It also has the advantage of not building a return list which is just thrown away after getting the first item. List::Util is part of the standard distribution for 5.8.0 and should be an easy install on previous versions, as well. And there's several other useful functions in there as well, not to mention the compainion module Scalar::Util.

bbfu
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Fearless on my breath