how can I do that ?

Well, regular expressions don't substitute contents of files. Neither does the substitute operator, which is probably what you meant. You'll need to read the file into memory in its entirety, apply the substitution, then write the text back to disk.

for my $qfn (@qfns) {
    my $text;
    {
        open(my $fh, '<', $qfn)
            or die("Can't open file \"$qfn\": $!"\n);
        local $/;
        $text = <$fh_in>;
    }

    $text =~ s///;

    {
        open(my $fh, '>', $qfn)
            or die("Can't create file \"$qfn\": $!"\n);
        print($fh $text);
    }
}
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can I do for multiple files simultaneously

You'd need to spawn processes or threads for that, but there's really no point to process the files simultaneously. I have a feeling you meant something different than what you said.

Something like this, for example (UNTESTED):

my $dir = '/some/path/to/files';

local @ARGV = glob "$dir/*";  # get all files in $dir

local $^I = '.bak';  # modify files "in-place"
                     # non-empty string required for Windows

while ( <> ) {  # loop through all files

    s/regular expression/replacement text/g;  # make modification

    print;  # save the result

    }
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