this should get you started, I kept it flexible so that you can adjust it. 

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

[download]

edit

In case you are sure that it's always exactly 3 digits, you can also use a hardwired regex, with a lookup array

s/(No.) (\d)(\d)(\d)/$1 \\$nums[$2]-$nums[$3]-$nums[$4]/g


  DB<94> $_ =" some text No. 345 other text No. 123 end text"

  DB<95> p
 some text No. 345 other text No. 123 end text
  DB<96> s/(No.) (\d)(\d)(\d)/$1 \\$nums[$2]-$nums[$3]-$nums[$4]/g

  DB<97> p
 some text No. \three-four-five other text No. \one-two-three end text

[download]

update

after reading the OP again, please provide an SSCCE clarifying input and expected output.

Cheers Rolf
(addicted to the Perl Programming Language :)
Wikisyntax for the Monastery

In reply to Re: multiple-pass search? by LanX
in thread multiple-pass search? by propellerhat

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