open IN, "d1.txt" or die "No such file:$!\n";
open OUT, ">ABC.txt" or die;
while ( <IN> )
{
    chomp $_;
    $a[ $l ] = $_;
    ++$l;
}

my @array = qw( doxorubicin is a potent anti-estrogen drug and tamoxifen );

for ( $i = 0; $i < $l; ++$i )
{
    for $z ( @array )
    {
        if ( $z =~ /\b\w^$a[$i]$\b/i )
        {
            $z =~ s/$z/<span>$z<\/span>/;
            $i++;
            print "@array ";
        }
    }
}