Sure, you can use a copy, to be declared outside of the BEGIN block:

use strict;
use warnings;
use FindBin 1.51 qw( $RealBin );

my $untainted_bin;

BEGIN { ($untainted_bin) = $RealBin =~ /(.+)/; }

use lib "$untainted_bin/../lib";

use MyLib;
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Thanks. This is beautiful. I improved it a little. Probably more portable with File::Spec.

use FindBin 1.51 qw( $RealBin );
use File::Spec;
my $lib_path;
BEGIN {
    $lib_path = File::Spec->catdir(($RealBin =~ /(.+)/msx)[0], q{..}, 
+'lib');
}
use lib "$lib_path";
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