my %cache;
sub first_line
{
    my ($filename) = @_;

    unless (exists $cache{$filename})
    {
        open( my $fh, $filename ) or die "Cannot open '$filename': $!\
+n";
        $cache{$filename} = <$fh>;
    }

    return $cache{$filename};
}
[download]

Instead of checking for the existence of the cached value, I check for the non-existence of the cached value. If it doesn't exist, I populate it. That way, I always return the cached value and the logic is slightly easier to work with.

On a sidenote, I would look at Memoize. It's a module that's built to do exactly what you're trying to do.

Update: Changed return to die as per jdporter's comment that reading an empty file gives back undef.

I would make the following additional change: If the open() fails, throw an exception (die), rather than returning a value which is indistinguishable from a valid first line. (An empty string is what you'd get if the file contains 0 bytes.)

Huh. Never knew that. It actually returns undef, not the empty string, which is what return; will send back to the caller. Updated.

------
We are the carpenters and bricklayers of the Information Age.

Then there are Damian modules.... *sigh* ... that's not about being less-lazy -- that's about being on some really good drugs -- you know, there is no spoon. - flyingmoose

I shouldn't have to say this, but any code, unless otherwise stated, is untested

my %cache;
sub first_line
{
  my $filename = shift;
  return $cache{$filename} if exists $cache{$filename};

  my $f = new IO::File $filename, "r" or die "file not found";
  
  my $line = <$f>;
  $cache{$filename} = $line;
  return $line;
}
[download]

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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And for those too lazy to write it by hand there's always Memoize and Aspect::Memoize.