What you want is probably:

my $code = 'sub { my $arg = shift; warn $arg; }';
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Update: Since the above looks alot like your original post, here is more.

I've never used Safe but my doc's indicate that you need a namespace as the first arg to Safe->new.

use Safe;

my $code = 'sub { my $arg = shift; warn $arg; }';

my $safe = Safe->new();

my $ssub = $safe->reval( $code) || die ;

$ssub->( "hello$/" );
&$ssub( "hello$/" );
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Be well,
rir