my $line = '<br>  2<br>   3<br>   4';
my $temp = $line;

while ($temp =~m#(<br>)(.+?)( \d+)#gsi)
{
my $full= $&;
my $text=$1;
my $space = "$2";
my $digit = "$3";
my @arr=();
(@arr) = $space =~s#( )# #gsi;
my $nbsptext = "&nbsp;" x "@arr";
$line =~s#$full#${text}${nbsptext}${digit}#gsi;
$full="";
}

print $line;

output:
<br>&nbsp; 2<br>&nbsp;&nbsp; 3<br>&nbsp;&nbsp; 4