# Loop over all files with a .log suffix
foreach my $fn (<*.log>)
{
    # Open the file, if possible, and read it all into @f
    open I, $fn or warn("Couldn't open $fn: $!"), next;
    my @f = <I>;
    close I;
    
    # Go through it a line at a time
    for (my $i = 0; $i < @f; $i++)
    {
        # If you find "hello" anywhere in the line,
        # Back up two lines and print if possible
        if ($f[$i] =~ /hello/)
        {
            print $f[$i-2] if $i > 1;
            print $f[$i-1] if $i > 0;
            print $f[$i];
        }
        # Note, if you don't care about "undefined value"
        # warnings, print the three elements without any
        # condition.
    }
}