The following code is untested, but probably has a 90-95% chance of working.
open(LOG, "source.txt") or die "Error: $!";
my @lines = <LOG>;
my $cnt = 0;
foreach my $line (@lines)
$line =~ m/(\d+\s+)(.+)/;
$lines[$cnt] = "$1 (Birthday) $2 - singer";
open(LOG, "> source2.txt") or die "Error $!";
print LOG join("\n", @lines);
"Age is nothing more than an inaccurate number bestowed upon us at birth as just another means for others to judge and classify us"