The "pev" function returns a value. What you need is "set_pev". Your line of code has no effect.
If that doesn't work, pretty sure SetFOV is supported in HL.
Code:
static iSetFOV
if(!iSetFov)
{
iSetFov = get_user_msgid("SetFOV")
}
message_begin(MSG_ONE, iSetFov, _, id)
write_byte(get_pcvar_num(g_pCvarFov))
message_end()