$line = '<br>  2<br>   3<br>   4';
$line =~ s#(?= )#&nbsp#gsi;
while ($line =~ s#(&nbsp) (&nbsp)#$1$2#gsi) { }
print $line;

o/p:
<br>&nbsp&nbsp 2<br>&nbsp&nbsp&nbsp 3<br>&nbsp&nbsp&nbsp 4