Regex in perl and variable name

I have some issues with a regular expression in perl. I'm trying to add a string at the beginning of a another string (in fact, insert a string at the beginning of the name of a file). What I want is check before inserting that string if the file already begins by it.

This is the code I have:

if ($ficheroSinExt !~ m/^$strCadena/){
    # if it doesn't exist at the beginning, I insert it...
    $ficheroSinExt = $strCadena . " " . $ficheroSinExt;
}
else{
    print "---->It already exists!!!\n";
}

I'm testing it with two filenames with only one containing [Perl] at the beginning ("[Perl] File1.pdf" and "File2.pdf"), and $strCadena contains [Perl]. I end up adding [Perl] for both files, so their new names are "[Perl] [Perl] File1.pdf" and "[Perl] File2.pdf".

I think the problem comes from the ^$strCadena of the match operator, but I don't arrive to work-around it. Could you please give me a hand?

Thanks in advance, Diego