my $Rm = 0xAAAAAAAA;
my $Rd = $Rm >> 31; # any shift amount from 0 top 31 for 32 bit data. 

$Rd |= 0xffffffff << (32 - 31); #it can be (32 - any shift amount)
$Rd = $Rd & 0xffffffff; #mask bits more than 32

printf "Rd = 0x%x\t Rb = %x\n",$Rd, $Rm;