President Barack Obama on Friday said the "whole nation is grieving right now" over the mass killings Thursday at the post, where 30 people were wounded.
The president ordered U.S. flags at the White House and other federal agencies to be flown at half-staff until Veterans Day, Nov. 11, as a tribute to those who lost their lives.
The suspected shooter, Maj. Nidal Malik Hasan, was shot by a police officer and remains hospitalized. The motive for the shootings is unclear.
Perry's order applies to all U.S. and Texas flags under the control of the state.