Right idea but you aren't using anything to anchor the pattern, so you would also match 'JohnABC, CDE, 14'

@arr1 = ("0000007 | John | ABC.txt | 42","0000014 | Jane | XYZ.txt | 3
+4","0000017 | Jessica | GHI.txt | 21", etc);
@arr2 = ('0000007', '0000014');

my $re = join('|',@arr2);
my @arr3 = grep /^(?:$re)\s/, @arr1;

print join("\n",@arr3),"\n";
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