s/./ do_smth( $& ) /eg;
m/.(?{ do_smth( $& ); })(*FAIL)/;
do_smth( $& ) while m/./g;

##</code><code>##

#!/usr/bin/perl

use strict;
use warnings; 

print "# The need of clear borders of the elements:\n";
print "## Without borders (wrong):\n";
"1 23 456" =~ m/\d+(?{ print "[$&]" })(*FAIL)/;
print "\n";
print "## With borders (correct):\n";
"1 23 456" =~ m/\b\d+\b(?{ print "[$&]" })(*FAIL)/;
print "\n";
print "## Alternative (correct):\n";
"1 23 456" =~ m/\d+(*SKIP)(?{ print "[$&]" })(*FAIL)/;
print "\n";

my @A = ( 1 .. 3, 'abc', 'zz', 79, 444 );
my $A = join ',', @A; # ',' -- one reserved character;
m/[,]/ and die "Elem '$_' of \@A contains separator '$&'!\n" for @A;

print "# SIMPLE LOOPING through an array:\n";
print "## NON-DESTRUCTIVE:\n";

for( my $i = 0; $i < @A; $i ++ ){
	print "[$A[ $i ]]";
	}
print "\n";

for my $A ( @A ){
	print "[$A]";
	}
print "\n";

$A =~ m/
	\b([^,]+)\b
	(?{ print "[$1]" })
	(*FAIL)
	/x;
print "\n";

print "## DESTRUCTIVE:\n";

for( my $i = 0; $i < @A; $i ++ ){
	$A[ $i ] .= 'X';
	print "[$A[ $i ]]";
	}
print "\n";

chop for @A;

for my $A ( @A ){
	$A .= 'X';
	print "[$A]";
	}
print "\n";

chop for @A;

$A =~ s/
	\b([^,]+)\b
	/ $1 . 'X' /gex;
print $A =~ s/\b([^,]+)\b,?/[$1]/gr;
print "\n";

$A =~ s/X//g;

print "# LOOPING through an array by evaluating several (2-3) consecutive elements:\n";
print "## NON-DESTRUCTIVE:\n";

for( my $i = 0; $i < @A - 1; $i ++ ){
	print "[$A[ $i ]-$A[ $i + 1 ]]";
	}
print "\n";

for my $i ( 0 .. @A - 2 ){
	print "[$A[ $i ]-$A[ $i + 1 ]]";
	}
print "\n";

$A =~ m/
	\b([^,]+)\b,
	\b([^,]+)\b
	(?{ print "[$1-$2]" })
	(*FAIL)
	/x;
print "\n";

# ----
for( my $i = 0; $i < @A - 1; $i += 2 ){
	print "[$A[ $i ]-$A[ $i + 1 ]]";
	}
print "\n";

for my $i ( grep $_ % 2 == 0, 0 .. @A - 2 ){
	print "[$A[ $i ]-$A[ $i + 1 ]]";
	}
print "\n";

$A =~ m/
	\b([^,]+)\b,(*SKIP)
	\b([^,]+)\b
	(?{ print "[$1-$2]" })
	(*FAIL)
	/x;
print "\n";

# ----
for( my $i = 0; $i < @A - 2; $i ++ ){
	print "[$A[ $i ]-$A[ $i + 1 ]-$A[ $i + 2 ]]";
	}
print "\n";

for my $i ( 0 .. @A - 3 ){
	print "[$A[ $i ]-$A[ $i + 1 ]-$A[ $i + 2 ]]";
	}
print "\n";

$A =~ m/
	\b([^,]+)\b,
	\b([^,]+)\b,
	\b([^,]+)\b
	(?{ print "[$1-$2-$3]" })
	(*FAIL)
	/x;
print "\n";

# ----
for( my $i = 0; $i < @A - 2; $i += 2 ){
	print "[$A[ $i ]-$A[ $i + 1 ]-$A[ $i + 2 ]]";
	}
print "\n";

for my $i ( grep $_ % 2 == 0, 0 .. @A - 3 ){
	print "[$A[ $i ]-$A[ $i + 1 ]-$A[ $i + 2 ]]";
	}
print "\n";

$A =~ m/
	\b([^,]+)\b,(*SKIP)
	\b([^,]+)\b,
	\b([^,]+)\b
	(?{ print "[$1-$2-$3]" })
	(*FAIL)
	/x;
print "\n";

# ----
for( my $i = 0; $i < @A - 2; $i += 3 ){
	print "[$A[ $i ]-$A[ $i + 1 ]-$A[ $i + 2 ]]";
	}
print "\n";

for my $i ( grep $_ % 3 == 0, 0 .. @A - 3 ){
	print "[$A[ $i ]-$A[ $i + 1 ]-$A[ $i + 2 ]]";
	}
print "\n";

$A =~ m/
	\b([^,]+)\b,
	\b([^,]+)\b,(*SKIP)
	\b([^,]+)\b
	(?{ print "[$1-$2-$3]" })
	(*FAIL)
	/x;
print "\n";

print "## DESTRUCTIVE:\n";

# ----
for( my $i = 0; $i < @A - 2; $i ++ ){
	$A[ $i ] .= $A[ $i + 1 ] gt $A[ $i + 2 ] ? 'X' : 'Y';
	print "[$A[ $i ]-$A[ $i + 1 ]-$A[ $i + 2 ]]";
	}
print "\n";
s/[XY]// for @A;

for my $i ( 0 .. @A - 3 ){
	$A[ $i ] .= $A[ $i + 1 ] gt $A[ $i + 2 ] ? 'X' : 'Y';
	print "[$A[ $i ]-$A[ $i + 1 ]-$A[ $i + 2 ]]";
	}
print "\n";
s/[XY]// for @A;

$A =~ s/
	\b([^,]+)\b
	(?=
	,\b([^,]+)\b
	,\b([^,]+)\b
	)
	/ my $new = $1 . ( $2 gt $3 ? 'X' : 'Y' ); print "[$new-$2-$3]"; $new /gex;
print "\n";
$A =~ s/[XY]//g;

print "# 'TRIANGLE' LOOPING through an array (loop in loop):\n";

for my $i ( 0 .. @A - 2 ){
	for my $j ( $i + 1 .. @A - 1 ){
		printf "%10s", " [$A[ $i ]-$A[ $j ]]";
		}
	print "\n";
	}

$A =~ m/
	\b([^,]+)\b
	.*?
	\b([^,]+)\b
	(?{ printf "%10s", " [$1-$2]" })
	(?=$)
	(?{ print "\n" })
	(*FAIL)
	/x;

print "# 'RECTANGLE' LOOPING through two arrays (loop in loop):\n";

my @B = @A[ 2 .. 4 ];
my $AB = $A . ';' . join ',', @B; # ',' and ';' -- two reserved characters;
m/[,;]/ and die "Elem '$_' of set { \@A, \@B } contains separator '$&'!\n" for @A, @B;

for my $i ( 0 .. @A - 1 ){
	for my $j ( 0 .. @B - 1 ){
		printf "%10s", " [$A[ $i ]-$B[ $j ]]";
		}
	print "\n";
	}

$AB =~ m/
	\b([^,]+)\b
	.*;
	.*?
	\b([^,]+)\b
	(?{ printf "%10s", " [$1-$2]" })
	(?=$)
	(?{ print "\n" })
	(*FAIL)
	/x;

##</code><code>##

# The need of clear borders of the elements:
## Without borders (wrong):
[1][23][2][3][456][45][4][56][5][6]
## With borders (correct):
[1][23][456]
## Alternative (correct):
[1][23][456]
# SIMPLE LOOPING through an array:
## NON-DESTRUCTIVE:
[1][2][3][abc][zz][79][444]
[1][2][3][abc][zz][79][444]
[1][2][3][abc][zz][79][444]
## DESTRUCTIVE:
[1X][2X][3X][abcX][zzX][79X][444X]
[1X][2X][3X][abcX][zzX][79X][444X]
[1X][2X][3X][abcX][zzX][79X][444X]
# LOOPING through an array by evaluating several (2-3) consecutive elements:
## NON-DESTRUCTIVE:
[1-2][2-3][3-abc][abc-zz][zz-79][79-444]
[1-2][2-3][3-abc][abc-zz][zz-79][79-444]
[1-2][2-3][3-abc][abc-zz][zz-79][79-444]
[1-2][3-abc][zz-79]
[1-2][3-abc][zz-79]
[1-2][3-abc][zz-79]
[1-2-3][2-3-abc][3-abc-zz][abc-zz-79][zz-79-444]
[1-2-3][2-3-abc][3-abc-zz][abc-zz-79][zz-79-444]
[1-2-3][2-3-abc][3-abc-zz][abc-zz-79][zz-79-444]
[1-2-3][3-abc-zz][zz-79-444]
[1-2-3][3-abc-zz][zz-79-444]
[1-2-3][3-abc-zz][zz-79-444]
[1-2-3][abc-zz-79]
[1-2-3][abc-zz-79]
[1-2-3][abc-zz-79]
## DESTRUCTIVE:
[1Y-2-3][2Y-3-abc][3Y-abc-zz][abcX-zz-79][zzX-79-444]
[1Y-2-3][2Y-3-abc][3Y-abc-zz][abcX-zz-79][zzX-79-444]
[1Y-2-3][2Y-3-abc][3Y-abc-zz][abcX-zz-79][zzX-79-444]
# 'TRIANGLE' LOOPING through an array (loop in loop):
     [1-2]     [1-3]   [1-abc]    [1-zz]    [1-79]   [1-444]
     [2-3]   [2-abc]    [2-zz]    [2-79]   [2-444]
   [3-abc]    [3-zz]    [3-79]   [3-444]
  [abc-zz]  [abc-79] [abc-444]
   [zz-79]  [zz-444]
  [79-444]
     [1-2]     [1-3]   [1-abc]    [1-zz]    [1-79]   [1-444]
     [2-3]   [2-abc]    [2-zz]    [2-79]   [2-444]
   [3-abc]    [3-zz]    [3-79]   [3-444]
  [abc-zz]  [abc-79] [abc-444]
   [zz-79]  [zz-444]
  [79-444]
# 'RECTANGLE' LOOPING through two arrays (loop in loop):
     [1-3]   [1-abc]    [1-zz]
     [2-3]   [2-abc]    [2-zz]
     [3-3]   [3-abc]    [3-zz]
   [abc-3] [abc-abc]  [abc-zz]
    [zz-3]  [zz-abc]   [zz-zz]
    [79-3]  [79-abc]   [79-zz]
   [444-3] [444-abc]  [444-zz]
     [1-3]   [1-abc]    [1-zz]
     [2-3]   [2-abc]    [2-zz]
     [3-3]   [3-abc]    [3-zz]
   [abc-3] [abc-abc]  [abc-zz]
    [zz-3]  [zz-abc]   [zz-zz]
    [79-3]  [79-abc]   [79-zz]
   [444-3] [444-abc]  [444-zz]

##</code><code>##

#!/usr/bin/perl

use strict;
use warnings; 

my @A = ( 1 .. 3, 'abc', 'zz', 79, 444 );
my $A = join ',', @A;

print "# With 'undef's:\n";

for my( $i, $j, $k )( @A ){
	print "[$i-$j-$k]";
	}
print "\n";

for( my $i = 0; $i < @A; $i += 3 ){
	print "[$A[ $i ]-$A[ $i + 1 ]-$A[ $i + 2 ]]";
	}
print "\n";

print "Without 'undef's -- no stepping out of an array:\n";

for my $i ( grep $_ % 3 == 0, 0 .. @A - 3 ){
	print "[$A[ $i ]-$A[ $i + 1 ]-$A[ $i + 2 ]]";
	}
print "\n";

$A =~ m/
	\b([^,]+)\b,
	\b([^,]+)\b,(*SKIP)
	\b([^,]+)\b
	(?{ print "[$1-$2-$3]" })
	(*FAIL)
	/x;
print "\n";

##</code><code>##

for my (...) is experimental at <...>
# With 'undef's:
Use of uninitialized value <...>
Use of uninitialized value <...>
[1-2-3][abc-zz-79][444--]
Use of uninitialized value <...>
Use of uninitialized value <...>
[1-2-3][abc-zz-79][444--]
Without 'undef's -- no stepping out of an array:
[1-2-3][abc-zz-79]
[1-2-3][abc-zz-79]