my $x = qq(1st line\n2nd line\n3rd line\n);

(my $y = $x) =~ s/^\d/_/g;    # useless use of /g
print qq("$y");    # output:"_st line\n2nd line\n3rd line\n" *)

($y = $x) =~ s/^\d/_/mg;
print qq("$y");    # output:"_st line\n_nd line\n_rd line\n"
($y = $x) =~ s/.\d/_/sg;
print qq("$y");    # output:"1st line_nd line_rd line\n"

($y = $x) =~ s/.\d/_/smg;    # useless use of /m
print qq("$y");    # output:"1st line_nd line_rd line\n"