You can index lists from the end. Untested:

my ($str1, $str2, $str3) = (split ' ', $line)[-3, -2, -1];
print if $str1 eq $str2 && $str2 eq $str3;
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Or as a one-liner (also untested):

perl -ane 'print if $F[-3] eq $F[-2] && $F[-2] eq $F[-1]' data-file
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