I've written a small routine that recognizes some image file formats and returns the width and height. But you're much better off using Image::Size...

# By max.maischein@econsult.de
#
# Simple recipe to create proper width= and height= attributes
# for <IMG> tags
    sub PNGsize( $ ) {
      # Returns a 2 element array containing the width and height of t
+he image
      my $Buffer = shift;
      my ($Width, $Height) = ( undef, undef );        # Assume failure

      if ($Buffer =~ /IHDR(.{8})/) {
        # Now save the numbers
        my $Data = $1;
    
        ($Width, $Height) = unpack( "NN", $Data );
      } else { print "'$Filename' is no PNG image (or has weird format
+)\n"; };

      return ($Width, $Height);
    };

    sub GIFsize( $ ) {
      my $ID;
      my $bytes = shift;
      my ($Width, $Height) = (undef, undef);

      if ($bytes =~ /^GIF8/) {
            ($ID, $Width, $Height) = unpack( "A6SS", $bytes );
      } else {
        print "imgsize:GIF:No GIF header found\n";
      };

      return ($Width,$Height);
    };

    sub JPGsize( $ ) {
      my $buffer = shift;
      my ($Width, $Height) = (undef, undef);
          my $SOF = pack("CC", 0xFF, 0xC0 );
          if ($buffer =~ /$SOF...(....)/ ) {
            $buffer = $1;
            ($Height,$Width) = unpack( "nn", $buffer );
          } else { print "imgsize:JPeG:No SOF header found\n"};  
      return ($Width, $Height);
    };

    sub img {
      my $Filename = shift;
          my $alt = shift || "";
      my $attributes = shift || "";
          my $ID;

      $alt =~ s/(.+)/ alt=\"$1\" /;

      my $result;
      my $absdir;

      if (!defined( &dirname )) {
        use File::Basename;
      };
      $absdir = dirname( $inputfile );
      my( $absname ) = build_path( $absdir, $Filename );
      #print $absname, "\n";

      my ($Width, $Height) = (undef, undef);
      my $SizeFunc = undef;                    # assume failure

      my $Buffer = "";

      if ( open( IMGFILE, "<" . $absname )) {
        binmode IMGFILE;
        read( IMGFILE, $Buffer, 2048 ) or print "Error reading from '$
+absname' : $!\n";
        close( IMGFILE ) or print "Error closing '$absname' : $!\n";
      } else {
        print "Error opening '$absname' : $!\n";
        $Buffer = undef;
      };

      if (defined( $Buffer )) {
        if ($Filename =~ /\.gif$/i) {
          $SizeFunc = \&GIFsize;
            } elsif ($Filename =~ /\.jpe?g$/i) {
          $SizeFunc = \&JPGsize;
            } elsif ($Filename =~ /\.png$/i) {
          $SizeFunc = \&PNGsize;
            } else {
           print "\nUnknown file extension for image : $imagename (ign
+ored)";
            };
        if (defined( $SizeFunc )) {
          ($Width, $Height) = &{$SizeFunc}( $Buffer );
        };
      };

      $attributes .= ( $Width ? " width=\"$Width\"" : "" ) . ( $Height
+ ? " height=\"$Height\"" : "" );

          $result = "<IMG src=\"$Filename\"" . $alt . $attributes . ">
+";
      return( $result );
    };
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