Your last print has a typo :-) your logic is backwards :-) and its simpler IMO to say:

my $add_at_end;
while (<IN>) {
 $add_at_end = 1 unless s/str1/str2/;
 print OUT;
}
print OUT "text" if $add_at_end;
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Update:And I had a typo also. Fixed mine first :-)