Short answer: read perlsec.

Long answer: @spec comes from file so it gets tainted. $tmp1 and $tmp2 are derived from @spec so they become tainted too. And $width with $height become tainted because they are derived from $tmp1 and $tmp2.

If you want to unaint $width and $height your code should look like:

sub getfilesize{
    my @spec=();
    open FH, $file;
    for (0..9){$spec[$_] = getc(FH);}
    close FH;
    my $tmp1 = unpack("H2",$spec[7]).unpack("H2",$spec[6]);
    my $tmp2 = unpack("H2",$spec[9]).unpack("H2",$spec[8]);
    $width = unpack('s',pack('s',hex($tmp2)));
    ($width) = $width =~ /(\d+)/;
    $height = unpack('s',pack('s',hex($tmp1)));
    ($height) = $height =~ /(\d+)/;
    }
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BTW if you are looking for simplicity just use Image::Size:

use Image::Size;
my ($width, $height) = imgsize($file);
($width) = $width =~ /(\d+)/;
($height) = $height =~ /(\d+)/;
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P.S. It is unrelated to your question but you are using bad coding practices in your code. You are using global variables without need: to pass parameters to sub ($file) and to return data from the sub ($width and $height). It is better to rewrite you sub this way:

sub getfilesize{
    my $file = shift;
    my @spec=();
    open FH, $file;
    for (0..9){$spec[$_] = getc(FH);}

    close FH;
    my $tmp1 = unpack("H2",$spec[7]).unpack("H2",$spec[6]);
    my $tmp2 = unpack("H2",$spec[9]).unpack("H2",$spec[8]);
    my $width = unpack('s',pack('s',hex($tmp2)));
    ($width) = $width =~ /(\d+)/;
    my $height = unpack('s',pack('s',hex($tmp1)));
    ($height) = $height =~ /(\d+)/;
    return ($width, $height);
    }

# example of sub call
my ($width, $height) = getfilesize('/path/to/file.gif');
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--
Ilya Martynov (http://martynov.org/)