"replaces a blank" (or, "replaces with a blank", I assume you mean). That's very pertinant information. Something is fishy there:

$ perl5.8 -e '$x = "a b c d"; $a = "blah"; $b{$a} = "foo"; $x =~ s/a b
+/$b{$a}/; print "$x\n"'
foo c d
[download]

Works fine here ... so, yes, there's something incomplete here. Perhaps you should post a complete test line that you expect to be modified by all of this so we can take a closer look.