In the 12th Patala (Chapter) of the Matrika Bheda Tantram, Goddess Chandika asks Lord Shiva that when Mantra initiation immediately results in bliss why in certain cases it results in madness and talkativeness among disciples.
To which Lord Shiva's reply is that it is due to a Dosha called "Mantracchanna". Basically there are some wrong ways of doing mantra japa that can affect the disciple in a negative way.
There are 8 kinds of Mantraccchanna doshas as given below.
Mantracchannam pravakshyAmi srunu devi samAhitA |
abhaktischakshare bhrAntir-luptash-chinnas-tathaiva cha ||
dirghashcha kathanam swapne tu chAshtadhA smritah |
naiva siddhih syAt kalpa-kotishatairapi ||
vA cheti bhrAtyA cha vAtulah |
nasho bhavet kila ||
hrasyocchAre vyAdhi-yukto dirgha-jApe
Lord Shiva said:-
I'm telling you what is Mantracchanata. Listen attentively. Abhakti,
Akshara-bhrAnti, LuptatA, ChinnatA, Hrasva, Dirgha, Kathana and
Swapna-Kathana ---- these 8 Doshas are known as MantracchanatA.
(Mantra japa done with) Abhakti does not give Siddhi even in crores of
eons; Whether the mantra is this or that --- this is called BhrAnti
and it results in vatulatA (talkativeness).
Mantra japa with Varna-lopa (alphabets missing from the mantra)
results in loss of intelligence; with Varna-cheda (that is one Varna
is missing from a compound Varna) results in death. If "long
alphabets" are pronounced as "short" and vice versa that lead to
diseases and destruction of wealth, respectively.
Sri MAtrikA Bheda Tantram; Chapter 12; Verses 42-45
Abhakti -- lack of devotion; Akshara-BhrAnti, LuptatA, ChinnatA, Hrasva and Dirgha are already explained; Kathana is revealing the mantra to somebody else and Swapna-Kathana is revealing the mantra in dreams to somebody else.
All these are the Doshas which render the Japa futile and affect the chanter in a negative way.
Since your only concern is about whether pronouncing Hrim as Hirim is right or not then it is not. Hrim is Ekakshara or one-letter but hirim is two-lettered. Here the compound letter Hri is missing from the mantra and new letters Hi and Ri are introduced. So, here one or more of the aforementioned Doshas are involved.