You might want to look through the Class::DBI and Ima::DBI documentation, specifically the section called "Dynamic Database Connections" in Class::DBI. You can define your own db_Main() method that returns your DBI connection rather than using set_db() directly. I've never done this, but this should work without a hitch. That is,

package MyClass;
use base 'Class::DBI';

# Stuff....

sub connect {
    # ....
}

sub db_Main() {
    return connect;
}
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