Probably the downvote because, as indicated (albeit somewhat indirectly), the response concerns unix grep(1), not perl grep, so the comma is not needed. Were it to be in perl, and had the file been read into an array using @file = <>, then grep /\[l\]/, @file would be correct, but from the command line, it ain't.
However, the original response is also wrong: since we're on the command line, the slashes are read as part of the regular expression, not as a pattern delimiter, so what you actually want is either
or (note single quotes)
open FH, "filename" or die "couldn't open file.\n";
@filename = <>;
print grep /\[l\]/,@filename;
You will note which of them is simpler.
% grep '\[l\]' filename
If God had meant us to fly, he would *never* have give us the railroads.