Frenchman Antoine Deltour, a former employee of auditing firm PricewaterhouseCoopers (PwC), is on trial along with two other people over the leaks and faces up to 10 years in jail.
"I feel a certain pride in having contributed to these important advances to taxation in Europe," Deltour told the court in the tiny EU duchy of Luxembourg as he gave evidence for the first time.
"One may hope that the outrage aroused (by the leaks) resulted in concrete political action."
The documents were originally used for a 2012 report on French public television, but only really exploded onto the world stage two years later with the huge "LuxLeaks" release of all 30,000 pages into the public domain.
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The files showed how Luxembourg granted "sweetheart" deals that saved firms including Apple, IKEA and Pepsi billions of dollars in taxes, at a time when Jean-Claude Juncker, now head of the European Commission, was prime minister.
Perrin, who is accused of violating business secrets and of encouraging Deltour to leak the papers, told reporters at the trial he "neither gave orders nor was a thief".
A third person, former PwC employee Raphael Halet, told the court last week he had leaked documents after deciding to do his duty as a citizen because he was "shocked" by Luxembourg's huge tax breaks for multinational firms.
The trial is expected to end around May 10.