Although perlprint asked only about blanks and letters, sometimes punctuation creeps in. The third alternative handles all non-blanks and avoids capturing:

>perl -wMstrict -le
"my $string = 'W h a t  H o ?  N o  G o .';
 (my $sqz1 = $string) =~ s{ [ ] \b }{}xmsg;
 print qq{'$sqz1'};
 (my $sqz2 = $string) =~ s{ \b [ ] }{}xmsg;
 print qq{'$sqz2'};
 (my $sqz3 = $string) =~ s{ [ ] (?![ ]) }{}xmsg;
 print qq{'$sqz3'};
"
'What Ho ? No Go .'
'What Ho?  No Go.'
'What Ho? No Go.'
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Update:  (?![ ]) vice  (?=\S) in third example.