Regular expressions can be used as "first-class-citizens"
(i.e. created and pre-compiled at run-time):

  DB<1> $pat='ATH.*?in';

  DB<7> $re=qr/$pat/ r

  DB<8> x $re
0  (?-xism:ATH.*?in)
   -> qr/(?-xism:ATH.*?in)/
  DB<9> x ref $re
0  'Regexp'
  DB<10> open(F,"</home/olaf/.bash_profile")

  DB<11> x grep /$re/, <F>
0  'PATH=.:$HOME/bin:$PATH:/usr/local/ocs-2.3f.beta/bin
'
  DB<12> 

qr// precopiles its argument as a regular expression. See perlop for
details.

For also choosing the options string at runtime, the syntax
"qr/(?$opts:$pat)/" appears to work:


  DB<148> $pat = 'ath.*?in'; $opts = 'i'

  DB<152> x $regexp=qr/(?$opts:$pat)/
0  (?-xism:(?i:ath.*?in))
   -> qr/(?-xism:(?i:ath.*?in))/
  DB<153> x 'ATHABCDIN' =~ /$regexp/
0  1
  DB<154> 

This is (somewhat) documented in perlre.