The code looks correct. On a stylistic note, I would write it slightly differently. 
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.

------
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

In reply to Re: Cache Subroutine Return Value by dragonchild
in thread Cache Subroutine Return Value by souravdas

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