All I can say is that it works fine for me. That sure is a lot of preceeding whitespace, but I get everything nonetheless. Maybe try stripping the whitespace? Other than that, try giving us some more information if you can?

print retrieve_url( 'http://www.finasta.lt' );

# note - don't use the parentheses after the sub name
#        we're not needing prototypes here

sub retrieve_url {
    my $url = shift;

    my $ua = LWP::UserAgent->new();
    my $res = $ua->get( $url );

    if ( $res->is_success() ) {
        my $content = $res->content();
        $content =~ s!\A\s+!!;

        return( $content );
    }
    else {
        die( "retrieval error: ", $res->status_line() );
    }
}
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