Your attempt at reading the file into an array won't work. You're only reading the first line (not even a paragraph). Another mistake in the code is that, in your if-statement, you use assignment (=) where you want a string comparison (eq). You should also switch on warnings and strict mode, especially if you're not yet entirely sure what you're doing.

Perl has a paragraph input mode which is activated by setting the variable $/ to an empty string. (See perlvar for more.) That lets you process a file by paragraphs as you read it.

Here is a revised version of your code (untested):

my $KWD = shift;
my $file = 'input.txt';
open my $in, '<', $file or die "Can't read '$file': $!";
local $/ = '';
while ( my $para = <$in> ) {
    my ( $first) = split /:/, $para;
    print $para if $first eq $KWD;
}
[download]

I have kept your approach using split() to isolate the part before ":", but a regular expression simplifies things:

/^$KWD:/ and print while <in>;
[download]

Anno