Sonething simple like this does the job:

my $find = quotemeta 'bla.bla.bla';

while(<DATA>) {
    if ( m/($find\s*=\s*)(.+)/s ) {
        my $begin = $1;
        my $change = $2;
        $change =~ s/ /\\ /g;
        print $begin.$change;
    }
    else {
        print;
    }
}
__DATA__
# don't change me
bla.bla.bla = c:/Some Windows/Path With/Spaces In/It
# don't change me
# don't change me
# don't change me
[download]

Which will print:

# don't change me
bla.bla.bla = c:/Some\ Windows/Path\ With/Spaces\ In/It
# don't change me
# don't change me
# don't change me
[download]

cheers

tachyon

s&&rsenoyhcatreve&&&s&n.+t&"$'$`$\"$\&"&ee&&y&srve&&d&&print