Doesn't this do that directly? $a = $b; The conditional seems unnecessary since it has no effect.

Take this values of $a and $b:

$a = 12;
$b = 2;
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From his words:

if $a does not match $b

I assume he wouldn't want to put the 2 in $a, as $a already matches $b.

why not:

$a = $b if($a ne $b);
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You should not be using $a and $b however, as they are "special" variables used by sort

You should not be using $a and $b however, as they are "special" variables used by sort

Doesn't matter. sort localizes $a and $b.

$a = 1;
print((sort { $b cmp $a } qw( a b c z y x )), $/);
print($a, $/);  # Prints "1"
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Does too matter. Try this code on perl 5.8.x.

my $a = 1;
print((sort { $b cmp $a } qw( a b c z y x )), $/);
print($a, $/);  # Prints "1"
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$a=$b if ($b ne $a);

how can i do this another way, like a:

+=,*=,.=,||=,..., etc

ps: because my variable $a is very long, for example:

$a{aaaaaaa1}{aaaaaaa2}{aaaaaaa3}=$b if ($b ne $a{aaaaaaa1}{aaaaaaa2}{aaaaaaa3});

Thanks beforehand!

Just do:

$a = $b;
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The if clause is fairly redundant (unless you're concerned about one of the variables being a blessed object which overloads stringification).

package Cow { use Moo; has name => (is => 'lazy', default => sub { 'Mooington' }) } say Cow->new->name

It's almost indentical to your subject:

Replace $a with $b if $a ne $b

$a = $b if $b ne $a;
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which simplifies to:

$a = $b;
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Or maybe you wanted

$a = $b unless $a =~ /$b/;
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There are plenty of solutions already mentioned, but I thought I'd mention something about the code that you tried:

$a !~ s/$b/$b/;

There is no !~ when dealing with substitution, only with matching. If there was no match in a substitution, it didn't find anything to replace, and so won't do anything. You can do odd things with negative assertions, but I would assume you probably wanted to match:

$a !~ m/$b/;

But, we get into the question of 'match' ... in this case, if $a contains $b, the expression will be true. if you want to ensure that $a didn't have any extra characters, you'd want to match from the beginning of the string to the end:

$a !~ m/^$b$/;

But if $b contains any regular expression meta characters (parens, brackets, braces, asterisks, periods, question marks, etc), odd things may occur that you didn't want to happen. So, if you don't want that, you use:

$a ne $b

If you are looking for $b within $a, and there's a chance of meta characters, then you'll want to use a regex, but escape any character in $b that might not be safe.

unless ($a =~ /$b/) {
  $a = $b;
}
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foreach $file (@filelist) {
  ($dir, $name) = split '/', $file;
  $dir =~ s/$currentdir//;
  printf("%s  %s\n", $dir, $name);
  $currentdir !~ s/$dir/$dir/;
}
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DIR1  FILE1
      FILE2
      FILE3
DIR2  FILE4
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Once you've cleared $dir, it's useless to compare $currentdir to it. Maybe you want something like:

my $printed_dir = '';
foreach $file (@filelist) {
  my ($dir, $name) = split '/', $file;
  printf("%s  %s\n", ($dir eq $printed_dir ? '' : $dir), $name);
  $printed_dir = $dir;
}
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---

Caution: Contents may have been coded under pressure.

There are at least a few different (and sensible) ways to do what you want in that "foreach" loop. Personally, I'd opt for turning the list into a hash of arrays, so that it is structured in a way that is consistent with what the output should be:

my %dirhash;
for my $file ( @filelist ) {
    my ( $dir, name ) = split '/', $file;
    pushd @{$dirhash{$dir}}, $name;
}
for my $dir ( sort keys %dirhash ) {
    print "$dir\t", join( "\n\t", @{$dirhash{$dir}} ), "\n";
}
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Another way, more similar to your original idea, would be to keep track of what the last printed directory was, and only print the directory name when it changes from the previous:

my $lastdir = '';
for my $file ( @filelist ) {
    my ( $dir, $name ) = split '/', $file;
    if ( $dir ne $lastdir ) {
        print "$dir";
        $lastdir = $dir;
    }
    print "\t$name\n";
}
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You shouldn't use a regex to split the filename from the path. Better take File::Basename for that.

use File::Basename;
my ($name,$path) = fileparse($fullname);
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holli, /regexed monk/