I just did a quick test of opening a small file 500 times using method A, then grepping, and method B and using a match. Results of my benchmark:

Method A:
1 wallclock secs ( 0.16 usr + 0.02 sys = 0.18 CPU)

Method B:
0 wallclock secs ( 0.04 usr + 0.03 sys = 0.07 CPU)

Results may vary, try it yourself to see what you get. Not very surprising since Method A will need to populate an array, the loop through it and match. Method B just does the match.

UPDATE: Out of curiosity I did Method A withought building an array and simply grepping from the open filehandle. Small difference:
0 wallclock secs ( 0.12 usr + 0.03 sys = 0.15 CPU)

Cheers,
KM