while($file = readdir BIN && $file=~/*$wordToBeReplaced*/) { 
		foreach $file (@NAMES) { 
			$file = HERE IS WHERE I WANT TO CHANGE ONLY   PART OF THE FILENAME WHEN I RENAME THE FILE 
			# change $newname 
			rename($file, $newname) or   
			warn "Couldn't rename $file to $newname: $!\n"; 
		} 
    }

    while($file = readdir BIN && $file=~/*$wordToBeReplaced*/) {  
		foreach $file (@NAMES) {

opendir(BIN,"path/to/directory") or die "$!"; 
my @files = grep {/$wordToBeReplaced/} readdir BIN; 
close BIN;

opendir(BIN,"path/to/directory") or die "$!" 
my @files = grep {/$wordToBeReplaced/} readdir BIN; 
close BIN; 
foreach my $oldfile (@files) { 
   my $newname = $oldfile; 
   $newname =~ s/searchpattern/replacepattern/; 
   rename($oldname,$newname) or    
   warn "Couldn't rename $oldname to $newname: $!\n"; 
}

rename("path/to/$oldname","path/to/$newname")

opendir(BIN,"path/to/directory") or die "$!"  
my @files = grep {/$wordToBeReplaced/} readdir BIN;  
close BIN;  
foreach my $oldname (@files) {  
   my $newname = $oldname;  
   $newname =~ s/searchpattern/replacepattern/;  
   rename($oldname,$newname) or     
   warn "Couldn't rename $oldname to $newname: $!\n";  
}

my $var = 'sleeping': 
my $replace = 'eat'; 
my $search = 'sleep'; 
$var =~ s/$search/$replace/; 
print $var; # prints "eating";

$foo = 'foo'; 
$bar = 'bar'; 
 
print $foo; # right 
print "$foo"; # works but is wrong 
print '$foo'; # wrong 
$foo = "$foo$bar"; # right because you are making a new string 
$foo .= $bar; # also right 
$foo = $foo . $bar; # also right 
$foo =~ /"$search"/$replace/; # definetly wrong