Some of the responses thus far won't properly handle a filename that evaluates to false (filename is '0' perhaps). Since you are using $self may I assume you are writing an object method? If so, perhaps you want to provide a get/set method to the file attribute and just call that appropriately within your other method:

sub whatever {
    my $self = shift;
    my $file = $self->file(@_);
    # ... do stuff
}

sub file {
    my $self = shift;
    $self->{file} = shift if @_;
    $self->{file};
}
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my ( $self, $file ) = ( $_[0],
                        ( $_[0]->{file} = $_[1] || $_[0]->{file} ) );
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my $self = shift;
$self->{file} = shift || $self->{file};
# or...
$file = $self->{file} = shift || $self->{file};
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even better (i think):

my $self = shift;
$self->{file} = shift if @_;
my $file = $self->{file}; # or just leave this out and use $self->{fil
+e}
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my $self = shift;
my $file = $self->{'file'} = shift || $self->{'file'};
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That way works for me but I find this way of writing it to be more easily grokkable:

sub file {
    my $self = shift;
    @_ ? $self->{file} = shift 
       : $self->{file};
}
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update hmm, sorry, that doesn't do exactly what you want I suppose. But you could always substitute $self->{file} or even $self->file() in place of $file through the rest of the method ... but, as my physics instructor never said, "whatever applies sufficient surface tension underneath your watercraft to maintain it above the liguid-gas boundary."

perl -e 'print "How sweet does a rose smell? "; chomp ($n = <STDIN>); 
+$rose = "smells sweet to degree $n"; *other_name = *rose; print "$oth
+er_name\n"'
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my ($self, $file) = ($_[0],$_[0]->{file}||=$_[1]);
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Unfortunately, that only sets $file to the second argument passed if $self->{file} isn't already set (to a true value, but I assume any filename passed will evaluate to true), which doesn't seem to be what princepawn wants (which is to set $file to be the second value passed if passed, or to be the value of $self->{file} otherwise).

Maybe what you want is for the call in the program to obj->file("Filename"); to store the filename internally, and my $fn = obj->file(); to return it.

If so, your code needs to look more like :

sub filename
{
   my ($self, $file) = @_;

   $self->{file} = $file if $file;

   return $self->{file};
}
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I don't like the $self->{'file'} = shift || $self->{'file'}; idea, since setting something to itself when you don't need to is less clear.
--
Brovnik