>perl -wMstrict -le
"my $needle = 'mo4\dy13\hr19\\';
 my $h1 = my $h2 = '\yr2012\mo4\dy13\hr19\min23\sec51';
 ;;
 print qq{'$h1'};
 $h1 =~ s/\Q$needle\E([^3]*3)/${needle}new/;
 print qq{'$h1'};
 ;;
 print qq{'$h2'};
 $h2 =~ s{ \Q$needle\E \K [^3]* 3 }{new}xms;
 print qq{'$h2'};
 ;;
 $h1 eq $h2 or die 'oops...';
"
'\yr2012\mo4\dy13\hr19\min23\sec51'
'\yr2012\mo4\dy13\hr19\new\sec51'
'\yr2012\mo4\dy13\hr19\min23\sec51'
'\yr2012\mo4\dy13\hr19\new\sec51'