use strict;
my $str = 'Раз СЛОВО2 два СЛОВО.
Раз СЛОВО два СЛОВО2 три СЛОВО.
Раз СЛОВО два СЛОВО2.';
my $regex = qr/(?<=[.?!]|^)(?:СЛОВО2()|СЛОВО()(?:\1())?|[^.?!])++\2(?!\3)/p;
my $subst = ' ЗАМЕНА';
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