$re = "(?:$flags:$core)";

That doesn't work: too many  ':' (colons): 

c:\@Work\Perl\monks>perl -wMstrict -le
"my $string = '/.*uba$/i';
 ;;
 my $flag = qr{ [msixpodual] }xms;
 ;;
 my $convertable =
 my ($core, $flags) = $string =~ m{
   \A \s* / (.*?) / ($flag*) \z
   }xms;
 die qq{bad rx: '$string'} unless $convertable;
 ;;
 my $re = qq{(?:$flags:$core)};
 print $re;
 print 'A: match' if 'uba' =~ $re;
"
(?:i:.*uba$)

[download]
Something like this works: 
c:\@Work\Perl\monks>perl -wMstrict -le
"my $string = '/.*u\/ba$/i';
 ;;
 my $flag = qr{ [msixpodual] }xms;
 ;;
 my $convertable =
 my ($pattern, $modifiers) = $string =~ m{
   \A \s* / (.*?) / ($flag*) \z
   }xms;
 die qq{bad rx: '$string'} unless $convertable;
 ;;
 my $rx = qr{(?$modifiers)$pattern};
 print $rx;
 print 'A: match' if 'u/Ba' =~ $rx;
 ;;
 my $ry = qr{ \A foo $rx }xms;
 print $ry
 ;;
 print 'B: match' if 'foolubatU/bA' =~ $ry;
 print 'C: match' if 'FoolubatU/bA' =~ $ry;
"
(?^:(?i).*u\/ba$)
A: match
(?^msx: \A foo (?^:(?i).*u\/ba$) )
B: match

[download]
Note that:

Give a man a fish:  <%-{-{-{-<

In reply to Re^2: creating qr from existing regex by AnomalousMonk
in thread creating qr from existing regex by Bobert1234

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