build_exclude_list() if $exclude_list_file;
build_exclude_dir() if $exclude_dir_file;

##</code><code>##

sub build_exclude_list {
    my $file = shift;
    return unless defined $file;
    open my $exclude_files, '<', $file or die "Can't open $file: $!";
    # ...
}
# ...

build_exclude_list ($exclude_list_file);

##</code><code>##

sub build_exclude_list {
    # ...
    return \%exclude_list;
}

my $exclude_list = build_exclude_list ($exclude_list_file);