use strict;
use warnings;

use List::Util q{first};

my $patt1   = q{ABC};
my $patt2   = q{XYZ};
my $replace = q{999999};

my $string  = <<EOD;
kjdfjdfXYZewfkfABClkjfef
sahasjABCsjhksfhABCsjsjfs
oreweouABCkerjeXYZewfkfABClkjfef
xcvmvbbbvABCdjfABCjsdjfsdf
jjnnjfABDjfXYZdjdjABCjfdkjfABClsfj
isosiXYZcsfsjfABChfdhgfABCyeryerXYZffjfs
EOD
print $string, q{-} x 25, qq{\n};

my @patt1Posns = ();
push @patt1Posns, ( pos( $string ) - length $patt1 )
   while $string =~ m{\Q$patt1}g;
my @patt2Posns = ();
push @patt2Posns, ( pos( $string ) - length $patt2 )
   while $string =~ m{\Q$patt2}g;

my @substituteSets = ();
foreach my $patt2Posn ( @patt2Posns )
{
    my $start = first { $_ < $patt2Posn } reverse @patt1Posns
       or next;
    my $end   = first { $_ > $patt2Posn } @patt1Posns
       or next;
    push @substituteSets, [ $start, $end - $start + length $patt1 ];
}

for my $raSubstituteSet ( reverse @substituteSets )
{
    substr
       $string,
       $raSubstituteSet->[ 0 ],
       $raSubstituteSet->[ 1 ],
       $replace;
}

print $string, q{-} x 25, qq{\n};

##</code><code>##

kjdfjdfXYZewfkfABClkjfef
sahasjABCsjhksfhABCsjsjfs
oreweouABCkerjeXYZewfkfABClkjfef
xcvmvbbbvABCdjfABCjsdjfsdf
jjnnjfABDjfXYZdjdjABCjfdkjfABClsfj
isosiXYZcsfsjfABChfdhgfABCyeryerXYZffjfs
-------------------------
kjdfjdfXYZewfkfABClkjfef
sahasjABCsjhksfhABCsjsjfs
oreweou999999lkjfef
xcvmvbbbvABCdjf999999jfdkjf999999hfdhgfABCyeryerXYZffjfs
-------------------------