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;
}

##</code><code>##

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