Sometimes being able to do it in "just a few lines" isn't the point (take Data::Page for example). It's having to always rewrite, correctly, those lines every time you want to solve this particular problem. In this case, the module makes for a more flexible solution, and it's a "few lines" (about 12 actually) that I don't have to rewrite. The downside is that it is about 20% slower, but that's a non-issue when response to any single query is imperceptible.

I can't see much justification for anything more complicated.

Optionally searching for strings instead of numbers?
Not having to have a fixed record size or not having to know what the recordsize is? (note: there is a set recordsize function in File::SortedSeek which claims to improve performance, but I could not see any difference in speed from using it)

Anyway, here's your function using F:SS instead:

use File::SortedSeek;

sub record_search {
    my ($key, $filename, $recordsize) = @_;
    my $fh = IO::File->new($filename, 'r')
        or croak("Could not open $filename: $!");

    # I don't see any difference in speed with the next line
    # File::SortedSeek::set_line_length($recordsize);
    numeric( $fh, $key);
    return File::SortedSeek::was_exact();
}
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Update: Looking at the answers below, I learned about Search::Dict, which would work just about as well, and has been in core perl since at least 5.6.1, but still supports my point about saving just a few lines.