use Image::ExifTool;
my $DPI = 300;
my $exif = Image::ExifTool->new;
$exif->SetNewValue("XResolution",$DPI);
$exif->SetNewValue("YResolution",$DPI);
$exif->SetNewValue("ResolutionUnit","inches");
$exif->WriteInfo($FILE)
	or warn "Failed to update EXIF in $FILE";

##</code><code>##

exiftool -XResolution=300 -YResolution=300 -ResolutionUnit=inches file.jpg