You will need to threads::shared::share it before you'll be able to assign it to a shared variable.

#! perl -slw
use strict;
use threads;
use threads::shared;

print "threads: $threads::VERSION threads::shared:$threads::shared::VE
+RSION";

{
    my $scalar = 0xdeadbeef;
    my $blessed = bless \$scalar, 'main';
    print 'Scalar: ', $blessed;
    my $shared :shared = &share( $blessed );
    print 'Scalar: ', $shared;
}

__END__
C:\test>t-blessed.pl
threads: 1.67 threads::shared:1.14
Scalar: main=SCALAR(0x1824624)
Scalar: main=SCALAR(0x1824624)
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For your case that should look something like:

my $capobj:shared;
$capobj = &share( Net::Pcap::open_live($dev,1024,0,0,\$err) );
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Or just

my $capobj:shared = &share( Net::Pcap::open_live($dev,1024,0,0,\$err) 
+);
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The ampasand on &share() is necessary to defeat the prototype.