Oracle's RPAD problematic via Perl's DBI module

The following code is returning an unexpected result. (Untested cut-n-paste; apologies)

Say the table is : students(name CHAR(8))

Entries in Students are : '        ' (8 blanks),
'bob     '   and     'dave    ' .

use strict ;
use DBI ;

my $st_name = " " ; # non-blank name
my $dstmt = $dbh->prepare("select count(*) from students

      where name = RPAD(?,8) ") ; # STMT AA
$dstmt->execute($st_name) or die "sql call failed";
my $num_entries = $dstmt->fetchrow() ;
$dstmt->finish ;

$num_entries should be set to 1; instead it is set to 0.
This problem occurs only for a blank string. If $st_name = "dave" , then the statements work properly.

If I run the query via sqlplus :
select count(*) from students where name = RPAD(' ',8) ,

    the result is 1, as expected.

I am using the getaround :
my $dstmt = $dbh->prepare("select count(*) from students

      where trim(name) = ? or (name = ' ' and trim(?) is null) ") ;
$dstmt->execute($st_name, $st_name) or die "sql call failed";

But still I am surprised why STMT AA (above) fails to return the expected result.