If the rest of your code uses OO mechanisms, you might want to recode new() so it will work as a class method rather than calling it as a non OO sub (ie. Main::new). The example below also checks that the passed in var is a valid DBI object. If you had done that in your code, you would have received an error as soon as you called the new method.

package Main;
use strict;

my $dbh;

sub new {
    my $class = shift;
    my $dbh = shift;
    
    UNIVERSAL::isa($dbh, 'DBI')
        or die "Main->new() was invoked without a valid DBI object";
}
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