Assuming you want to reinvent the wheel regardless of what jrsimmon says, your issue is that %8s does not mean 8 spaces followed by the string; rather it means set the minimum width of the field at 8, padding the front with spaces as necessary. For your purposes, you'd want something more like:

#!/usr/bin/perl
use strict;
use warnings;

my $spaces=0;
while (chomp(my $line=<DATA>)) {
    $line =~ s/^\s+|\s+$//g;
    if ($line =~ /}/) {
        $spaces-=4;
    }
    my $padded = ' ' x $spaces . $line;
    print "$spaces:$padded\n";
    if ($line =~ /{/) {
        $spaces+=4;
    }
}
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avoiding the sprintf all together. More info at sprintf - search for "(minimum) width".