This works for me:

  DB<5>  $object = "2-4 5-12"

  DB<6>  $object =~ s/([-\d]+)/\\$1/g

  DB<7> p $object
\2-4 \5-12
[download]

Or you could store the backslash in a variable and use that variable:

  DB<11>  $c = '\\';

  DB<12> p $c
\
  DB<13>  $object = "2-4 5-12"

  DB<14> $object =~ s/([-\d]+)/$c$1/g;

  DB<15> p $object
\2-4 \5-12
[download]