use strict;
my $str = 'including the [[Royal Danish Academy of Sciences and Letters|Danish Academy of Sciences]], [[Norwegian Academy of Science and Letters|Norwegian Academy of Sciences]], [[Russian Academy of Sciences]], and [[National Academy of Sciences|US National Academy of Sciences]].
including the Danish Academy of Sciences, Norwegian Academy of Sciences, Russian Academy of Sciences, and US National Academy of Sciences.';
my $regex = qr/\[{2}(?:(?:(?!]{2})[^|])+\|)*((?:(?!]{2})[^|])+)]{2}/p;
my $subst = '\\1';
my $result = $str =~ s/$regex/$subst/rg;
print "The result of the substitution is' $result\n";
Please keep in mind that these code samples are automatically generated and are not guaranteed to work. If you find any syntax errors, feel free to submit a bug report. For a full regex reference for Perl, please visit: http://perldoc.perl.org/perlre.html