Please note that the replacement part of a s/this/that/ is not a regular expression so the dots do not need to be escaped.

knoppix@Microknoppix:/media/sda1/perl/Monks$ perl -E '
> $str = q{6 - 10, 14 - 17, 19};
> $str =~ s{-}{..}g;
> say $str;'
6 .. 10, 14 .. 17, 19
knoppix@Microknoppix:/media/sda1/perl/Monks$
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I hope this is helpful.