Your code as posted: 

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 tamoxif
+en );

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 ";
        }
    }
}

[download]
My script is fine,

I don't see how you can say that.

You are trying to use the string "(5r)-6-(4-{[2-(3-Iodobenzyl)-3-Oxocyclohex-1-En-1-Yl]Amino}Phenyl)-5-Methyl-4,5-Dihydropyridazin-3(2h)-One" as a regular expression to match against the contents of @array but the elements of @array are all smaller than that string so they will never match.    The regular expression has to be able to fit inside the string.

You are using the regular expression /\b\w^$a[$i]$\b/i which says: Match a Word boundary followed by a single Word character followed by the beginning of the string followed by the contents of $a[$i] followed by the end of the string followed by a word boundary.    You can't match anything BEFORE the beginning of the string or AFTER the end of the string, there is nothing there to match.

In reply to Re: Invalid range [] by jwkrahn
in thread Invalid range [] by Nandini21

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