To answer your question directly, I would split off the file-reading bit from the HTML-generation bit (to avoid reading the file multiple times), then pass some arguments to your subroutine so it can be a little more flexible. Why make 4 nearly identical subroutines whose outputs differ in only 1 character? Pass in an argument!

my @countries = do {
    my $fh;
    open($fh => "countries.txt")
     ? map { /(.*):(.*)/ ? $1 : () } <$fh>
     : ();
};

my $dropdown = make_dropdown("dropdown", @countries);
my $another  = make_dropdown("another",  @countries);

sub make_dropdown {
    my ($form_name, @countries) = @_;
    
    ## generate and return a dropdown with <select name="$form_name">
}
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But a much better solution for your problem in general would be to just use the CGI modules's existing capabilities, including the (horribly-named) popup_menu routine which generates a dropdown. You are using CGI, right?

use CGI 'popup_menu';

my $dropdown = popup_menu("dropdown", \@countries);
my $another  = popup_menu("another",  \@countries, "USA"); ## set a de
+fault
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The benefit of this is that you can easily set the selected value of the dropdown.. And it's already been written and tested, etc.