use strict;
my $str = 'Hello @[Kenneth Auchenberg](user:1) its been long. Tell @[Pete Lacey](user:6) I will see him tomorrow';
my $regex = qr/@\[([^\]\s]+)([^\]]+)\]\([^\)]+\)/mp;
my $subst = '[$1$2](site.com/user/$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