What the error is telling you is that $_ is uninitialized at the line s/\s//g, which is the variable you are doing the substitution on, since you have not given it another (e.g. $variable =~ s/\s//g;

I think what you want is probably stomething like:

if ($line =~ /^%(.{1,100})$/){
 
 $name = $1;
 $name =~ s/\s//g;         #this is the variable I think you want to r
+emove spaces from. 

 print "EEEEEEEEEEEEEEEEEEEEEEEEEEEE: $name\n";
    }
    
    $outfile = "outfile['$name'].txt";
    open (OUTFILE, "+>>$outfile");
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On a side note unless the spaces will be in the center of the variable $name you might just want to anchor the search to the beginning (using ^)and the end (using $) as it is faster.

-enlil