The syntax error is because you don't have a block for your 'if' statement, only for the 'while'. You could get by without the block if you turned it into a modifier (see perlsyn), as follows:

print $_ if( $_ =~ m/$keyword/ );
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There's more to it than that, though, because you're using the assignment operator ('=', see perlop) in what appears to be a pattern match, hence the modification I made to your code in the above example. If @keywords is large, use Super Search to find examples of using an array in a pattern match. If it is reasonably small, you can use alternation (perlre):

my $pattern = join( '|', @keywords );
if( $_ =~ m/$pattern/ )
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Oh, and in addition to using strict and warnings, you may also find diagnostics helpful.

HTH

ikegami is absolutely right about escaping special characters, of course (see quotemeta). Thanks and ++ for pointing that out!

Update: full code example, with escaped characters

use strict;
use warnings;

my @keywords = qw( keyword1 keyword2 keyword3 );
my $pattern = join( '|', map quotemeta, @keywords );
print "pattern = [$pattern]\n";

# using 'if' as a modifier
while( my $line = <DATA> )
{
    print $line if ( $line =~ m/$pattern/ );
}

# using an 'if' block
while( my $line = <DATA> )
{
    if( $line =~ m/$pattern/ )
    {
        print $line;
    }
}

__DATA__
somelines...
somelines...
somewords...keyword1..somewords
somelines...
somewords...
keyword2...somewords...
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pattern = [keyword1|keyword2|keyword3]
somewords...keyword1..somewords
keyword2...somewords...
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