Adding a character to a line using Perl

perl -p -i -e 's/(.)$/$1$1/g' filename 

This changed my nonsense file: 

ghggk 
dethaks 
gjfkdld 
fyduftsdu 
flkgjd kflgjlk flkgjl f 

into a slightly different nonsense file: 

ghggkk 
dethakss 
gjfkdldd 
fyduftsduu 
flkgjd kflgjlk flkgjl ff