You could also simply try

my $x1 = $r << 32 | $t;
printf "\nThe value is 0x%x", $x1;   # 0x23400000345
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(though whether this works similarly depends of the build options of your perl)

P.S.: What exactly do you need this for?  Maybe bigint or Math::BigInt would help.

Update:

use bigint; 

my $r = 0x234;
my $t = 0x345;

my $x1 = $r << 32 | $t;

print $x1->as_hex;
[download]

while(1)
{
print"Thankyou...."
}
--$ugum@r--

BTW, note that using bigint indiscriminately can have a massive performance penalty.  So be sure to always localize it to exactly where you actually need it (by putting extra {...} blocks around those sections).

#!/usr/bin/perl
use strict;
use warnings;
use Benchmark qw(cmpthese);

{
    use bigint; 
    
    # code that needs bigint
    my $r = 0x234;
    my $t = 0x345;
    my $x1 = $r << 32 | $t;
    #print $x1->as_hex;

    # other stuff that doesn't...
    #   => unnecessarily slow!
    sub with_bigint {
        my $x = 1;
        $x = $x * 4 / 2 - 1 for 1..1000;
    }

} # end of bigint

sub no_bigint {
    my $x = 1;
    $x = $x * 4 / 2 - 1 for 1..1000;
}

cmpthese (-1,
    {
        with_bigint => \&with_bigint,
        no_bigint   => \&no_bigint,
    },
);

__END__
              Rate with_bigint   no_bigint
with_bigint 11.3/s          --       -100%
no_bigint   4073/s      35876%          --
[download]

Q   An unsigned quad value.
    (Quads are available only if your system supports 64-bit
    integer values _and_ if Perl has been compiled to support those.
    Causes a fatal error otherwise.)
[download]

I know that "perldoc -f pack"
why it shows this error "Invalid type 'Q' "?

--$ugum@r--