open (FH,"text.txt");
 @lines=<FH>;
 
 @patterns = qw/<string> line secondline/;
 @replace = qw/'<REPLACED>' 'line_R' 'secondline_R'/;

 foreach $line (@lines){
    chomp $line;
    for ($i=0;$i<=2;$i++){
	if ($line =~ /\B$patterns[$i]\B/i){
	   $line =~ s/$patterns[$i]/$replace[$i]/i;
	   push @repd,$line;
	}
    }
 }

 foreach $line (@repd){
    print "$line\n";
 }