DB<49> sub english_num { my ($pre,$num) = @_; my $eng = join "-", map {(qw/zero one two three four five six seven \
eight nine/)[$_] } split //,$num; return "$pre \\$eng"}

  DB<50> $txt =" some text No. 345 other text No. 123 end text"

  DB<51> $txt =~ s/(No.) (\d{3})/english_num($1,$2)/ge

  DB<52> say $txt
 some text No. \three-four-five other text No. \one-two-three end text