Well, I am not sure why you would want to do that. If you are trying to capture DBI warnings/errors, you can give the DBI handle an error handler:

$h->{RaiseError} = 1; # Turn all warnings into dies
$h->{HandleError} = sub { set_dberror(join('',@_)) };
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Any errors generated by the dbh will be passed to your sub.

If you really want to capture capture all warning for specific modules, you might try:

package TrapWarnings;

my %Modules;
BEGIN { $SIG{__WARN__} = \ &trapWarning; }

sub import {
     $Modules{caller} = 1;
}

sub trapWarning {
     my $caller = caller 1;
     if ($Module{$caller}) {
          set_dberror(join('',@_))
     } else {
          warn @_
     }
}

# Example use

package Foo;

use TrapWarnings;

warn "foo";
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Well that code is completely untested. The general idea is everytime you import your TrapWarnings package, it adds that package to the list of packages to trap warnings for. When a warn happens, it checks the original caller to see if its package is in the list. If so, it traps it, otherwise, just re-warns it.

This is far from ideal, but with creative use of the Carp module you might get where you want to be.