I've never researched that, but if I make building blocks to assemble regexes, I typically surround that with (?:), unless it's not going to matter. So, I'd write: 
my $bblock1 = "[a-f]";
my $bblock2 = "foo";
my $bblock3 = "(?:$bar|$baz)";

[download]
And then there's: 
my $bblock4 = "[a-i]";
my $bblock5 = "[A-I]";
my $bblock6 = "[0-9]";
my $bblock7 = "$bblock4|$bblock5|$bblock6";
   $bblock7 =~ s/\Q]|[\E//g;  # Make one range

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In reply to Re^3: Composing regex's dynamically by JavaFan
in thread Composing regex's dynamically by John M. Dlugosz

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