... how would we know that this code is doing caching activity?

I've seen some cases where people take the trouble to provide a report on caching effectiveness -- basically, you want to know how many times "first_line" was called, and of those times, how often you hit on a cached value.

That sort of thing is easy to add -- here's how it might look, adding to dragonchild's proposal, which is very clear and clean:

my %cache;
my ($ncalls, $nhits);
sub first_line
{
    my ($filename) = @_;
    $ncalls++;
    if (exists $cache{$filename}) {
        $nhits++;
    }
    else {
        open( my $fh, $filename ) or die "Cannot open '$filename': $!\
+n";
        $cache{$filename} = <$fh>;
    }

    return $cache{$filename};
}

END { print "first_line returned cached value $nhits times in $ncalls 
+calls\n" }
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Usually you'd want the report to be conditional on a run-time option -- e.g. the user includes a "-v" or "-d" flag when running the script from a command line to select "verbose" or "debug" mode.