I agree with previous replies that there is no point to using a 'one big regex' solution when the job can be done more clearly, even if with more statements.

That said, I could not understand why the  $match2 regex would not extract the  b named capture from  $input1 string: there is clearly a  'b=2' substring there to  b extracted.

A little instrumentation made the situation clear. The first  /.*?/ lazy expression and the  /(?<b>b=(\d))?/ lazy capture are immediately satisfied. That leaves the second  /.*?/ lazy expression and the final required expression at the end of the line to match. The second  /.*?/ tries to match with nothing, but then must backtrack (forward-track?) and consume all text (including the  'b=2' substring) until the final required expression matches in order to achieve an overall match.

This is made clear in the following (note the added named capture around the second  /.*?/):

>perl -wMstrict -le
"my $input1 = 'a=1 gibberish b=2 c=3';
 my $match2 =
   '^(?<a>a=(\d)).*?(?<b>b=(\d))?(?<gib>.*?)(?<c>c=(\d))$';
 print qq{input1 '$input1'};
 print qq{Using regex $match2};
 if ($input1 =~ m/$match2/){
   print qq{a '$+{a}'  b '$+{b}'  c '$+{c}'  gib '$+{gib}'}; }
 else { print qq{Input1: didn't match}; }
"
input1 'a=1 gibberish b=2 c=3'
Using regex ^(?<a>a=(\d)).*?(?<b>b=(\d))?(?<gib>.*?)(?<c>c=(\d))$
Use of uninitialized value in concatenation (.) or string at ...
a 'a=1'  b ''  c 'c=3'  gib ' gibberish b=2 '
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