Since you didn't use code tags around the data, those who try to check your code are at a disadvantage... which is a bad approach when asking for free help.

Then you assure us that the error messages are the "desired output."

So, obviously, problem solved. Moving on...

But maybe that's another mistake... so you get advice from Corion and RichardK to check the content of @t and, specifically, $t[7] which, thus far, you don't appear to have done since you tell us the "data in the 8th column that shows "HIGH", "LOW", "LRR" " but which we can't verify/troubleshooot based on anything you've shown... but which is NOT the case in your sample data, where those strings occur in the seventh column, and should be found in $t[6] if the only problem with your sample data is your formatting and html's rendering of the separators as single spaces.

#!/usr/bin/perl -w
use 5.014;

#992723

my @temp =<DATA>;

my $f=join("",@temp);
my @lastreco=split("\n",$f);

foreach  (@lastreco) {
    my @t= split("\t",$_);
    say "testing: $t[6]";
    if ($t[6] =~ /HIGH/ && /LOW/ && /LRR/) {
        print "matched: $1\n";
    }
}

=head unmodified sample data has spaces, not tabs

860414 31 290 28 299 00931.17 HIGH CL1122

860412 501 541 501 554 00977.18 LRR_4 CL0022

860415 501 541 511 564 00977.33 LOW CL0023 

=cut

__DATA__
860414    31    290    28    299    00931.17    HIGH    CL1122
654321    404    403    402    401    011.1111    ARR34    CL0024
860412    501    541    501    554    00977.18    LRR_4    CL0022
860415    501    541    511    564    00977.33    LOW    CL0023    
123456    404    403    402    401    011.1111    MED    CL0024
[download]

Output:

testing: HIGH
testing: ARR34
testing: LRR_4
testing: LOW
testing: MED
[download]

And no, this doesn't address why, even with the corrections above, you're not getting a match... but that answer will be found on numerous (that is, "many," but not "all") nodes here using the term "alternation" or in the documentation for logical operators.

Don't read the whole file into memory. Read it line by line instead:

while (<FILE>) {
 ...
 # $_ is the current line
}
[download]

Also, there is no need in the @t array: just slice the array returned by split:

if ((split "\t",$_)[7] =~ /^(?:HIGH|LOW|LRR)$/)
[download]

Also, your condition ($t[7] =~ /HIGH/ && /LOW/ && /LRR/) will never be true as long as one line never contains "HIGH", "LOW" and "LRR" at the same time.

If the words "HIGH", "LRR" and "LOW" can only appear in the column you're interested in (looks like it, given your data set), then try the following on the string that contains your file's entire contents:

use Modern::Perl;

my $data = <<END;
860414 31 290 28 299 00931.17 HIGH CL1122
860412 501 541 501 554 00977.18 MED CL0022
860412 501 541 501 554 00977.18 LRR_4 CL0022
860412 501 541 501 554 00977.18 SSA CL0022
860415 501 541 511 564 00977.33 LOW CL0023
END

say $data =~ /(?=.*HIGH)(?=.*LRR)(?=.*LOW)/s ? 1 : 0;
[download]

Output:

1
[download]

The regex effectively 'ANDs' the terms together.

First, the snippet of the file that you show is space-delimited, but you are splitting each line on the tabs.

Second, when counting elements in an array in Perl, you should start at zero, not one. HIGH/LRR/LOW is in the 7th column which would be element number 6 (or $t[6]) if the line is properly split.

Third, I don't think you mean to test that column 7 matches ALL of the keywords at once.

Desired output comes as ..

Use of uninitialized value $t7 in pattern match (m//) at ./program_2.pl line 14.

Use of uninitialized value $t7 in pattern match (m//) at ./program_2.pl line 14.

Use of uninitialized value $t7 in pattern match (m//) at ./program_2.pl line 14.

So, have you looked at the contents of @t? Have you verified that it contains what you want? Perl starts counting array indices at 0. Are you sure that the data in the eighth column is what you want?