On Solaris, changing LD_LIBRARY_PATH from within a process doesn't work, because the dynamic linker is caching the info. In other words, you have to set it before, e.g. in a shell wrapper, or some such. Fork-and-exec (as opposed to simply forking) should also work though, I suppose.

Update: As to re-exec-ing yourself, here's one way you could do it (tested on Solaris 10):

#!/usr/bin/perl

BEGIN {
    my $orahome = "/path/to/orahome";
    my $oralib  = "$orahome/lib";
    unless ($ENV{LD_LIBRARY_PATH} =~ /$oralib/) {
        $ENV{ORACLE_HOME}     = $orahome;
        $ENV{LD_LIBRARY_PATH} = $oralib;
        exec $0, @ARGV;
    }
}
use DBD::Oracle;
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