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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