my %o=(this=>33, 'th at'=>44, other=>55,'some thing'=>66,'or another'=>77,none=>88); 

while (my ($k,$v)=each %o){ 
 my $ModifiedK = $k;
 next unless  $ModifiedK =~s/\s/_/;
 delete $o{$k}; #This is SAFE !! RTFM.
 $o{$ModifiedK}=$v;
};

print qq(Mod: $_\t=>$o{$_} \n) for sort keys %o"