Just use concatenation if you want to capture all the lines.

my $output;

$PatternToMatch="^SELECT";
$tmpfile="pattern.txt";
open(MYFILE, "$tmpfile")|| die "Cannot create $tmpfile\n";
while (<MYFILE>) {
if (/$PatternToMatch/)
{
 #print   "$_";
 $output .=$_;
 }
}
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