If it's really one line,

my ($source_cont) = $CtRet =~ /^source cont=(.*)/m
   or die("Didn't find source cont\n");
[download]

If it's two lines as shown,

my ($source_cont) = $CtRet =~ /^source cont=("[^"]*")/m
   or die("Didn't find source cont\n");
$source_cont =~ s/\n/ /g;
[download]

Updated: Oops! Added missing m modifier.