package CommonPiece;
require Exporter;
our @ISA =qw/Exporter/;
our @EXPORT =qw/htmlStyle/;

sub htmlStyle {
  print "<style>...</style>";
}
1;
__END__

##</code><code>##

use CommonPiece;
htmlStyle; # this works

##</code><code>##

BEGIN {
  require CommonPiece;
}

htmlStyle; 
#Undefined subroutine &main::htmlStyle called at /...path/