#!/usr/bin/perl -n

if ( m@^[ \t]*([A-Za-z_]+):[ \t]*(.*)[ \t]*$@ )
{
    push @assigns, "\$$1=$2;";
    push @subst, "s/__$1__/\$$1/o;";
}

END
{
    print "BEGIN\n{\n    ", join( "\n    ", @assigns ), "\n\}\n\n";
    print join( "\n", @subst ), "\n";
}