#!/usr/bin/perl
use strict;
use warnings;
{
    package My::DBI::st;
    our @ISA = qw(DBI::st);

    sub prepare {
        my ($self, $str, @args) = @_;
        $self->SUPER::prepare("/* userid $ENV{USER} */ $str", @args);
    }
}

sub new_dbh {
    ...
    my $dbh = DBI->connect(...);
    return bless $dbh, 'My::DBI::st';
}